Mozilla security warning continuously popping up

I have mozilla version 21 on windows 8 and for the last 3 weeks I've noticed that I keep getting the following pop up message twice on each page:

Although this page is encrypted, the information you have entered is to be sent over an unencrypted connection and could easily be read by a third party.

on almost every page I visit. Google, facebook but not gmail and not some newspaper websites I read. How can I deactivate this? It's driving me crazy! I also noticed that there are lots of ads in pages were there didn't used to be. Even as I am writing this message, there are two square ads between the first menu bar and 'post new thread'. I even got an ad in the body of an email messageI was trying to write when I opened my work webemail page the other day. Help? I have avast and ran a scan, it found nothing. But there must be some king of problem here.....

Thanks a lot for your help!

 

download the add on add blocker which works great with mozilla.
no idea how it will run with 8 though but worth a try.

 

Excellent suggestion^^^ on this site switch to VB4-default style on the pull down at the bottom of any page. No-Script is also an excellent addition to Firefox; (but does not address this issue), but as MrDJ suggests not sure about win 8... I would use the add-on manager within FF for the most recent version.
IF the issue persists this may help: How do I disable "encrypted page/unencrypted connection" security warning? | Firefox Support Forum | Mozilla Support