Here's a quick and dirty Firefox port. I've only given it the barest amount of testing but it seems to work, although the sidebar open/close behaviour is baffling me. Maybe it will make sense to someone who uses Firefox.
The rules for loading your own addons in Firefox appear to be even stricter than in Chrome. Instructions are in the included README.md.
If someone would like to try it out then go for it. If it works properly I'll put it on GitHub too.
HumbleBundleDownloaderFirefox.zip