This is only indirectly related to Ubuntu or Linux, so I’ll keep it brief: FireFTP is probably one of the coolest plugins for Firefox I’ve ever seen.
I’ve never liked gFTP; I find it slow, clunky and counterintuitive. It also has some default startup options I think are too obscure to be useful. I only use it begrudgingly, because it seems to be one of the few “working” FTP options for Linux.
At the same time, I use SmartFTP at work, but it is so completely hamstrung by nag screens and built-in sabotage (like eradicating your favorites) that it’s hardly an option there, either.
Since I use Firefox at home and at work, the FireFTP extension is an absolute godsend. Straightforward, easy to figure out and it does the job well. Browser integration is … well, it’s already integrated. It’s open-source, cross-platform and actively maintained. And for the newcomers, it’s even easier than sudo aptitude install — just point Firefox at the extension page, and follow through.
P.S.: Works great in Iceweasel, too. ;)