It appears so, depending on where in Friesland you are. Page 149 here shows that in Heerenveen, for instance, almost half of the 16-18 year olds surveyed had Frisian as a home language, so I would guess they learned it as a primary language.
I think this is an important point for us English speakers. We are unusual in that most speakers of Indo-European languages have another major* language that is rather similar. We don’t. While Spanish and Italian, for example, are generally considered to be mutually unintelligible, if two speaker of those languages make a concerted effort to do so, they can largely make themselves understood. Good luck trying to make that happen between an English and Frisian speaker.
You have Hindi and Urdu, Russian and Ukrainian, Italian and Spanish, Swedish and Norwegian, Persian and Dari, etc. We have Scots, which is quote close to standard English, but it’s spoken natively by fewer than 200,000 people.
*How many English speakers are going to interact regularly with speakers of Scots or Frisian?
In my experience traveling in Sweden I found I could piece together a lot more written Swedish then I thought I’d be able to. Nothing too elaborate, but I understood a lot of signage, etc. I don’t recall English helping that much though. It was my very basic (enough to pass it as a mandatory subject in high school, but not to converse in any meaningful way) understanding of Afrikaans (a Dutch derivative) that gave me enough word roots and other similarities to get by.
There is a great similarity in the sentence intonation and accents in the North-East of England and in Hamburg: the first time I went to Germany was in Hamburg, and local TV was showing a play in Plattdeutsch, which I could only understand by thinking they were all Geordies trying to speak German!
I think Danish and Old English have a lot of cognates, but even Old English takes some work and training to read. I have had friends from all the Scananavian countries and you can occasionally guess the meaning.
Me, too. I spent quite a bit of time in Norway and Germany, and I often found the best way of understanding one language was to delve not so much into English, but into the vocabulary I knew of the other language.
Anecdote: I remember driving out of a parking garage in Germany, and seeing a button one was to push in order to raise the gate. Above the button was written “drucken” (umlaut of the “u”). I laughed a bit, thinking “trykk” in Norwegian (“y” is pronounced similar to “u”).
I thought Flemish was generally closest to English in that it borrows more English and French vocabulary than standard Dutch. I spent a handful of days in Belgium in 2001 and had to make a phone call to verify my flight home. The lady in the phone prompts wasn’t intelligible to me but about every third word was one I recognized. That was a lot more disconcerting than not understanding it at all.
I spent a solid two months in Denmark (with some jaunts to Sweden) in that same trip, and couldn’t understand a thing. While Danish, Swedish, and Norwegian are all written basically the same (substitute ø for ö and a few other things then you’re basically there), the Dane’s pronunciation is much different and sounds more like Klingon. It seems Swedes and Norwegians can talk to each other for the most part, but even if the Danes can understand Swedish and Norwegian, their pronunciation is so different it’s easier for everyone to just speak English.
The odd thing about those languages is that written Danish and Norwegian (Bokmal) are basically the same (it wasn’t that long ago that Norway was ruled by Denmark), but spoken Norwegian and Swedish are closer. Written Swedish vs Norwegian/Danish is a bit more than adjusting a few letters, as you say.
Danish with respect to English is remarkable for the large number of “false friends” in the vocabulary. One day I just paged through a small Danish-English dictionary and listed about 300 false friends. Many of which would cause embarrassing/hilarious mistranslations. I wish I still had that list.
There was (and still is) a bakery next to the train station where I stayed that in the morning had all their windows lined with danishes, breads, and all manner of breakfast pastries, much like you’d expect to see. However, in the evening it was all swapped out for the most immaculately decorated and sumptuous cakes, pies, and desserts you could possibly imagine. Many with tantalizing chocolate glazes 
I cry Uncle! UNCLE!! UNCLE!!!
Looks like you could gain serious weight just by walking past the windows/walls. 
Is a Danish Danish any different from a Non-Danish Danish?
Here’s an interesting theory I ran across a little while ago - Middle English, distinct from Old English, was anglicized Norse from the old Danelaw.
It does not. I married into a Danish family, and my daughter is also a native speaker. We speak 3 languages at home, and I find English and Spanish just as similar in its vocabulary.
There’s loan words (swear words in Danish are a bit quaint, lots of “fucks” and “shits” make it better), some Latin (gratis), some Greek (apothek), some cognates, and some hilarious false friends (fart). I can’t say my English helps that much.
The Danish “du” in “du kan gå” doesn’t sound like the English “you” in “you can go.”
Bah! This whole board is becoming recursive ;).
I’m Norwegian and work in a hotel. Every night a Dutch crew from an airline check in and I can’t help but overhear their conversation. I can grok about 20% of what they say, but I feel like I should be understanding 70-80%. Like I’m talking with someone in English with a super-thick accent. Or when someone you share more than one language with speaks to you and for a second you’re not sure which language they spoke in so you can’t parse the meaning.
Here’s a random sentence from a Danish newspaper from today:
Here’s the same sentence written in Norwegian:
Virtually identical, right?
Here’s the same sentence transliterated into English.
A few things stand out. Here’s the original sentence with every individual word I expect a native English speaker will linguistically grok intuitively underlined:
Not that many, right? But a native English speaker is probably familiar with the German word “arbeit” for work. Someone culturally familiar with the Nordic countries knows the Parliaments in Norway, Iceland and Denmark are called the Storting, the Allting and the Folketing respectively. Or at least recognize the name of the new, Danish PM and infer it.
If the native speaker have any further basic familiarity with German, they might also guess the similarity of “nye” and “neue”, maybe. “Med” and “mit”. “Zusammen” and “sammen.”
Then there’s the sentence structure. It translates virtually perfectly, unlike German. And there’s an end-quote mark, a word and a name so it makes sense that the word means “writes” or “says” because that’s a normal literary convention. And since we’re talking about parliament, we can infer that DF is probably a political party and that means “partier” probably just means parties.
And now the sentence looks like this:
This is just guesswork for me, so I’m curious if I’m close to the mark here.
Well, for a start, to a Dane it’s Viennese.
By whom? 
You barely need to make more effort than to communicate with a Spanish or Italian speaker from a different dialectal group: avoid dialectal terms (common), avoid speaking faster than a gun on full auto (common), and if you’re the Spanish speaker favor Latin roots over others (not common). And remember that burra means different things 