Firefox 4.0.x is no longer available. You can download the previous secure version which is v3.6.19 from here: Download Firefox v3.6.19

However, I suspect your problems are related to the adware you have installed which is constantly running in the background searching for ads to feed you. To terminate this behaviour, do the following.

1. Click the Firefox button, go to Add-ons and then click the Plugins link.
   
   
2. Disable the following:
   

• My Web Search
  
  
• Mindspark Toolbar Platform Plugin
  

Also, update Java either by using Windows Control Panel, or go to http://www.java.com/en/

If you decide to downgrade to v3.6.19, please follow instructions here to ensure a clean installation.

1. After you download the installation file via the above mentioned link, go to Add/Remove Programs, scroll down to "Mozilla Firefox" and remove it, choosing to keep your bookmarks, customizations etc., (don't checkmark the box).
   
   
2. Then reboot and delete the folder called "Mozilla Firefox" at this location: C:\Program Files\Mozilla Firefox
   
   
3. Finally run the installation file you downloaded to the desktop earlier.
   

Your bookmarks, customizations etc., are maintained in a different location and will become available to you again once you complete the installation.

Having said that, make a backup of your bookmarks as a precaution as follows:

1. Click the Firefox button, go to Bookmarks, then Show All Bookmarks to open the Bookmarks Manager.
   
   
2. Click the link to Import and Backup and then click Export HTML and save the file somewhere.

In my case it's McAfee Site Advisor that is not compatible with Firefox 5. I was trying to wait until early August to upgrade to Firefox 5, when McAfee say they will have fixed the problem, but Firefox upgraded without administrator permission, which was irritating. I stopped it many times, but when I upgraded ITunes, Firefox jumped in without asking!

Firefox 4 was working OK, of course, and aside from failing to support McAfee Site Advisor, Firefox 5 was OK for me.

@lancecowled

Are you aware that the McAfee Site Advisor add-on duplicates features that Firefox has had for over three years now?

http://www.mozilla.com/en-US/firefox/phishing-protection/

You may prefer McAfee's features and interface for Malware / Phising warnings over the default Firefox feature, but it's not like McAfee was adding something that Firefox doesn't already have. Different interface, with more "bells & whistles", and a different database supplier, but IMO that doesn't offset McAfee being known as "problematic" for many years, and Mozilla eventually "blocklisting" that add-on because it was wreaking havoc in the Firefox 5.0 betas, being the #2 cause of Firefox crashing.
http://kb.mozillazine.org/Problematic_extensions
https://addons.mozilla.org/en-US/firefox/blocked/
https://addons.mozilla.org/en-US/firefox/blocked/i25

My biggest peev was that google's toolbar is not supported on firefox 5, and its a necessity because I have many systems but also many users. Forefox by itself does not have the features I needed from the google toolbar (not even the sync cuts it) So thank you Xircal for supplying the older version it works best :)

You're welcome.

Could you please click the "Solved It" button next to the post which solved the problem for you? This will help others searching for a solution to the same question.

Thanks.