You say you just installed Win7. What was the previous OS? How ancient is this PC and what are its specs? Particularly, how much RAM memory?
Also, what’s up with the LensCrafter swindle?
Go to about:support in the URL bar. Click the Reset Firefox button. See if that fixes it. Most Firefox problems can be fixed that way. (You’ll lose your extensions, though.) if that doesn’t work, uninstall Firefox, and choose the option to delete all personal files when prompted. Then reinstall Firefox. If that doesn’t fix the problem, then the problem is not with Firefox.
And don’t ever uninstall Internet Explorer. Windows is often buggy and unstable without it. Just delete the icons and don’t use it.