I think Firefox 3.x has a good javascript engine - it's just not hardware accelerating the canvas, so all the textures and alpha blending are done in software.
Knowing the Opera folk they will come out with a much faster build in the coming months, same for firefox as firefix 4 looms.
OK it also runs slower on my Firefox than my Chrome. The real surprise was that you can scale the game size (like pictures) with Chrome using CTRL+ MOUSE_WHEEL
with firefox too
edit: with opera and IE9 too
My Firefox v3.6.13 will change text size but not image, nor the game window size.
Am I missing a setting? How did you get yours to change image size?
Develop games in your browser. Powerful, performant & highly capable.
I think that's just the browser's full-page-zoom feature. Nothing to worry about.