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. 😉
Motho ke motho ka botho 
FileZilla 3.0 is included with Feisty!
The one thing about fireftp that I didn’t like is the fact that it won’t work with drag and drop. It doesn’t work with gftp either.
Not only does it not work with drag and drop I also found out that it has a 4 GB download limit so I can not download any Linux DVD distros. That was the final thumbs down for me.
I have been using FileZilla for a while and I haven’t look back.
I must be using a different version; mine will drag and drop. I’ll try it at home and make sure it’s not just a quirk of the Windows version I have to use at work.
Edited: Well gang, I’m not sure if things changed since you tried it last, but I can drag and drop downloads with FireFTP in Firefox 2.0 on Windows XP and with Iceweasel 1.5.0.7 on Edgy. Maybe give it another try? (P.S.: I can’t speak to download size limits. If I find a 4Gb file I want to download, I’ll give it a try and report back. 😉 )