Personally, I'd say to just remove all of the add ons and see if it works then... If you haven't done this already anyways. If it turns out that the issue was from Add ons, just add each of those add ons back one by one, testing after each one to see which one caused it. This way you'll be able to keep you add ons except the one(or more) that is/are causing the issue.
If this isn't it, try checking your firewall settings and such, maybe blocking images from sites.
If you've ever messed around with ports/port forwarding, I've had issues once where I port forwarded and suddenly only certain sites would load... So if you've port forwarded or messed with ports any, go check those and see if anything funny's going on.
Possibly, probably not likely, but possibly could also be parental controls.
Could be proxy stuff as well, though I've never heard of that specific issue, I have heard of people messing with proxies and ended up with issues using some/all browsers. Personally I've never messed around with proxies, complete stranger to them. Meaning, if that is the issue, I'm stumped as to what you could do about it.
This is all I can think of personally... Good luck, hope any of this helps.
Typically and preferably a Window's user, so I am somewhat talking based off of knowledge from that, but from what I've noticed there isn't too much of a difference as far as issues go.
- SamuTheFrog