Firefox will remember your text size setting per site. At least it does for PC and I can't see why it would be any different for Imac. There are some differences between the PC and Linux versions, but I don't think that is one of them.

Now, if I could just get them to remember your color settings per site, I could turn off this grey when I don't need it. There are some cool sites that I miss good design because I over-ride the colors. But, with 80% or better of the internet pages being done in white or worse, some gawd awful color combination, I'll stick with my grey background until firefox covers me. Much easier on the eyes.