Do a malware check with some malware scan programs.
You need to scan with all programs because each program detects different malware.
Make sure that you update each program to get the latest version of the database before doing a scan.

• http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware
• http://www.superantispyware.com/ - SuperAntispyware
• http://www.safer-networking.org/en/index.html - Spybot Search & Destroy
• http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free
• http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page

See also "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked and What to do when searches take you to the wrong search website

I have the same problem. Specifically, it's that my browser tries to route through my old academic institution's proxy server - but since I don't work there anymore and thus no longer have a login, it's effectively blocking me from seeing the sites at all. I'm sure I set this up somehow years ago, but I can't for the life of me find where. I've looked at all the settings locations I can find, and even had the institution's tech support try to walk me through it. It seems ridiculous that I can't figure out how to turn this off... but I just can't seem to!

You can find the connection settings in Tools > Options > Advanced : Network : Connection

See "Firefox connection settings":

• Firefox can't load websites but other browsers can

I figured it out!! For me, at least, it was zotero!

Independent of what you have set under Tools > Options > Advanced : Network : Connection, zotero will recognize that given domains have been accessed via a particular proxy previously, and redirect for you. You can just go to the preferences in zotero and delete a particular proxy, or turn off the proxy redirects altogether.

See this thread: http://support.mozilla.com/en-US/questions/672293?s=proxy+redirect&as=s