Firefox (YES!) -- Wouldn't consider anything else.  I've written a few extensions for it, and it's generally *invaluable*.

GIMP (no) -- Totally inferior to Photoshop.  Perhaps it's because I have a fair bit of experience with
    Photoshop, and really use just about every tool it offirs, so it's painful when they're missing from
    GIMP.

Inkscape (yes) -- Love it, though I don't have any experience with others to say what's missing.