Framed sites broken (yopmail, gmane, gmaps, etc)
Since I've updated to Iceweasel 6.0-4 and subsequently 7.0~b2-1 ("b2" stands for "beta 2") the Torbutton add-on (1.4.1) is breaking the frame system. Reproducible with (e.g.) [0] (other pages with frames, I'm using, are behind logins and thus no good public examples ;)). Instead of loading the e-mail's content in the lower frame, a new tab/window (depending on your settings) is opened. Disabling Torbutton works around this bug.
Iceweasel 6.0-2 was the last known working version. Iceweasel is the same as Firefox, just the name was changed due to Mozilla making much ado about nothing. The Debian revisions (e.g. "-2" or "-4") indicate updates to the packaging and sometimes the inclusion of upstream commits/patches, for details about what changed, see [1].
[0] http://news.gmane.org/gmane.comp.video.mesa3d.devel [1] http://packages.debian.org/changelogs/pool/main/i/iceweasel/current/changelog and http://packages.debian.org/changelogs/pool/main/i/iceweasel/iceweasel_6.0-4/changelog