Comment #21 on issue 313 by mmikucionis: Gears website erroneously
reports "browser not supported" for Iceweasel
http://code.google.com/p/gears/issues/detail?id=313
I think it is hopeless to expect to support every Firefox derivation (I
also think it is stupid for web pages to rely on user-agent in the first
place), but anyhow...
Just change the user-agent string from Iceweasel to Firefox in about:config
and this will solve most of your compatibility problems, not just Gears.
The are two problems with this workaround though:
1) The version will not be incremented when Iceweasel is upgraded.
2) Mozilla may object for trademark violation or some other crazy legal
stuff, but hey, life is tough and then you die :-)