I decided to try both and see if either sucked any less than my last experience. The answer is no, but FF is FAR smaller. While my Opera installation has accumulated a decade's worth of bookmarks, skins, cookies, and extensions (including Greasemonkey, believe it or not), it is still smaller than a clean install of Firefox (and has everything I need and then some built in; for FF I need to add a bunch of extensions to even make it serviceable). But Chrome, for some completely absurd reason, is over 100 MiB – more than twice the size of FF, and three times the size of Opera my (or roughly 7 times the size of clean install of Opera, which was about 15 MiB the last time I did one).
EDIT: Finally found Chrome so I don't need to guess on its size. For some equally absurd reason, it installed itself in /users/fewilcox/appdata instead of Program Files (stupid programmers screwing with my folder tree; grrrr). It weighs in not at a bit over 100 MiB like when it was first released – it is now nearly THREE HUNDRED, yet still has only minimal features. Sheesh. My laptop simply can't waste that much space on something so mostly useless, so now I'm off to reclaim 300 MiB of space that is far better spent holding our CD collection once I finish ripping it.
So I went with FF simply because I didn't want to waste more drive space than I had to on a browser I only needed for one website. I just can't give up Fast Forward, Tab Stacking, or being able to cycle tabs in recently used orders, the way Windows does, instead of only in the order they are the tab bar (especially since the various projects I'm working on now mean I have more than 100 tabs at the moment; finishing one project up shortly, then I can close about a dozen of them).
On that note, I really wish they'd do with Roll20 what Google does with their various HTML 5 projects and simply warn you that your browser might not work instead of refusing to let you try. Websites telling me what browser to use annoys me so much I wouldn't have Kicked Roll20 had I known they did that. For the record, except for Wave, I've only ever had very minor issues with Google's HTML 5 stuff that said they won't work in Opera, so the same may be true of Roll20 but they won't let us find out.