Another not particularly obvious feature of Firefox is the ability to use Ctrl and the plus and minus keys from the numeric keypad to make the text bigger (in the case of Ctrl-+) or smaller (in the case of Ctrl–). This is probably the feature I miss the most when using Internet Explorer, because I don’t usually even notice when I resize the text at a site. This tip only works for text, not images.
You can also use the pulldown menu View->Text Size->Increase or View->Text Size->Decrease, but the Ctrl keys are much faster.