ColorZilla – has a colour picker for HTML colours, plus also highlights the ID and size of HTML elements on the page.
Nuke Anything – right click and get rid of elements on the page (i.e. that massive picture of a half naked woman etc.)
Adblock Filterset.G Updater – updates your adblock settings automatically
Flashblock – blocks Flash until you enable each individual Flash element – stops sound from automatically starting up etc.
Html Validator – validates that your pages are compliant to their doctype and highlights problems in the source view
IE View – for when you’re on Windows and you want / need to open a page in IE (for testing etc.)