captcha problem Rated 3 out of 5 stars
@WhoeverIam 3000: you could give gs.netsend.nl a try, it's not as popular as proxy.googlesharing.net and I haven't seen a captcha yet.
fix uploaded for "400 Bad Request" errors Rated 5 out of 5 stars
I fixed the "400 Bad Request" reply I'm getting now and then because of an extremely large cookie that is being sent. An updated add-on with this patch included is hosted at http://gs.netsend.nl. The particular patch itself can be seen at https://github.com/timkuijsten/GoogleSharing
Alternative proxy Rated 5 out of 5 stars
To overcome the captcha problem an alternative proxy has been setup at http://gs.netsend.nl. This site also hosts a modified plugin that has the Firefox maxVersion number bumped to 15.x and has this extra proxy added.