Does Firefox crash for you when you open a new window?
Report the crash using the built in tool, then browse “about:crashes” (shows you which crashes you reports), click the topmost link.
If frame #0 in “Crashing Thread” (bottom of the page) reads “libtracker-sparql-0.12.so.0.1205.0”, then this package is the culprit: tracker-miner-firefox
Delete it and Firefox should work again. If that helps in your case, you’ve been hit by this bug.
What’s this piece of crap do? From the web site:
If you don’t know, Nepomuk is the great technology that builds search indexes over anything on your disk. Unfortunately, it’s developed by people who believe quality in software is nice to have. So it tends to hog the CPU wasting your time, it can fill your harddisk with useless junk, and please, don’t run it on computers with more than 100 MB (that’s 0.1 GB) of disk space or more than 1’000 files; otherwise, it might never finish. If you try to search something, bring some time – useless results take a moment to come up with.
Don’t disable it either, or apps like Dolphin will be unhappy. It’s already beyond the abilities of these people to hide the UI elements for rating and tags when you try to protect yourself.
I opened a bug to have Nepomuk removed from KDE until it reaches alpha status but that bug was closed.