The ability to install Firefox on a USB memory stick and run it on any Windows computer (or Linux using Wine - works okay for me!) is one of the greatest and coolest things I have ever come across.
I have plugins for Google Bookmarks (personal) and del.icio.us (work - there are historical reasons for this) so that I can share bookmarks with my home FF on Linux, or access them if some prick of an IT officer tries to lock off the USB ports.
Thanks to full support for plugins I get crash restore, I get sensible handling of tabs, I get tab scrolling with the mouse wheel and tabs loading in the background.
I had forgotten quite how much I relied upon this as I shuttled between Uni and two different jobs until I forgot my keys, and left them hanging in the door at home when I slipped out without waking my partner, and therefore don't have my USB key.
I am now approximately five minutes, or one more IE7 crash, away from complete apoplectic rage, tantrum and going home. I have now seen more IE7 crashes than spam, all in one morning, and I've only been clocked on for 87 minutes so far. I haven't lost anything, or links, but it's been a close-run thing (and thank you to Blogger, too, for autosaving!)
Please, PortableApps.com, don't give up or fade away. I don't care if portable FF never gets the ability to update itself safely and I have to download and install the full file every time, I forgive it. I don't even care if I never need to install any other PortableApps.com app (although... Gimp might be handy...). So long as I have my FF with me at all times, with my settings and my plugins and working how I need it to in order to be productive, I will forgive a lot.
I will never, ever, forgive Microsoft for IE7.