Flash Player on Linux

I'm going to switch to Linux with my new computer. I've read something weird about Firefox not supporting Flash on Linux (Chrome seems to support it, but I wouldn't want to switch if possible). The only sites I remember that I visit that use Flash are Youtube (with which I have no problem because I suppose it'll work fine with HTML5 (it'll work even better, because now with flash player I can't use the HTML5-only half speed configuration) and minigame sites (which I suppose it won't work anymore, but that's no big deal). However, will this affect other sites I can't remember now? Just to be sure, is there any good alternative? (I've heard about Lightspark and Gnash, but they say that they aren't very good).