Have you tried a different browser?
I am rather surprised that you have these problems with Firefox, I’ve used it since it came out with virtually no problems.
I do use Chromium alongside it, mainly to keep email accounts separate which may be worth a try.
Do install uBlock Origin though, saves lots of hassle with adverts.
Also install Clam TK (from the repositories) and run a malware check. Unlikely that there will be anything other than Windows viruses.
Install Bleachbit and configure it - it comes in two versions, one for normal use to ditch cookies, temp files etc. in your ‘user’ profile, and a second ‘root’ profile to remove unwanted files from the system.
As an alternative to Ubuntu, try Linux Mint, which has a look and feel similar to Windows 7. Very easy to install and use.
I have both Ubuntu and Mint, four examples of which have recently been installed with ease. I keep a few cheap 4Gig USB dongles for just this purpose.