I’m not saying that your customisations are breaking Firefox, I’m saying that something in your profile has gone awry and is causing it to crash. I’ve not used the page colour customisation but it’d be worth trying a stock Firefox then dropping your customisations back in to see if that fixes it.

You can create a new profile by following these instructions: http://kb.mozillazine.org/Profile_manager#Linux

You can also see a list of crash reports from your install by going to about:crashes . These link through to the Mozilla crash log server, which often then links onto the appropriate bug in Bugzilla. You might find that what’s causing the crashes has been fixed in an upcoming release.