(In reply to comment #0)

The crash only happens when I upload images at:
http://commons.wikimedia.org/w/index.php?title=Special:Upload&uselang=fromwikimedia

Related reports about the crash led me to this:
http://crash-stats.mozilla.com/report/index/6cd3e443-fddd-446f-a2bd-b24ad2100128

That's a bug report against Firefox, and rightly so: Firefox should not allow a random web site to crash it, so if it does that's a bug in Firefox. Closing as INVALID for this reason: not a bug in MediaWiki.

Now, FF 3.6 error console also reports:
Error: $j is not defined
Source File: http://bits.wikimedia.org/skins-1.5/common/edit.js?urid=257z2
Line: 33

I don't know what this all means. I thought it's best to let you know.

This is probably not related, but it's annoying so I fixed it.