I use Mozilla and like it a lot. A lot of people also like Firebird, which is also made by the Mozilla folks.
Are there any compelling reasons to switch from Mozilla to Firebird? I guess Firebird is smaller and uses less resources, but if Mozilla runs fine do I gain anything going to Firebird?
Personally, I think the Jar of Pickles would win … 
Sorry for the smart-ass answer; I really don’t have anything useful to say …
I’ve been trying out Firebird for the last few weeks, and I can’t say I’ve noticed much difference to ‘classic’ Mozilla. The menu and toolbars take up less space, but apart from that there don’t seem to be any stand-out new features, and the memory footprint is about the same (on Windows, at least). Also, being pre-release, there are still some minor bugs.
Maybe when it goes to official release, which I think will be Mozilla 1.6, they will have managed to get the memory usage down, but right now I wouldn’t particularly recommend it over Mozilla.
I’ve been using it for a while. At least on my computer, it seems to load up faster than regular Mozilla. I also just prefer the way it feels. The last one is completely subjective.
I prefer Firebird; its what I am using right now. It seems a bit faster, and I like the interface more.
Also, does Firebird get a 1920’s style “Death Ray” for the battle?
Firebird is definitely faster, and the download is half the size. Being on dialup, the latter was a compelling reason to go with it. Optimized compiles for your processor, available at Mozillazine.org in the “Firebird Builds Forum,” will be even faster for you.
For more information about the bugs, fixes, and changes in recent Firebird nightly builds, see Burning Issues.