Advise you never allow CCleaner to have any access to any Thunderbird files and folders. CCleaner has a nasty habit of deleting various files that may incude session data, passwords etc.

Work through the following info at link: https://support.mozilla.org/en-US/kb/hyperlinks-in-messages-not-working

Extra checks: In computer 'Control Panel' > 'Default Programs' Check all the following:

'Set your default programs' Make sure Firefox is set as default browser

'Associate a file type or protocol with a program' make sure you have all the following using Firefox.

   .htm
   .html
   .mfp
   .shtml
   .xht
   .xhtml 

Protocols:

   FTP
   HTTP
   HTTPS 

'set program access and computer defaults' select: 'Custom' and click on drop down arrows

If eg:Firefox is not open: select 'Mozilla Firefox' and 'Enable access to this program' do not select any other option If Firefox is currently open then select 'Use my current Web browser' - but make sure there is only 'Firefox browser open and not eg: IE or Chrome as well. Personally I always close the program and then make sure it is selected in this section.

This solution did not help. Here is my problem specific. I can not click on the hyperlink in my Classmates.com email and land where it is supposed to. Instead it takes me to this link: https://search.yahoo.com/yhs/search?hspart=iba&hsimp=yhs-1&type=undefined_6017_FFW_US&p=email I have tried all the solutions suggested. This didn't start happening till about 2-3 weeks ago....before that it always went to where it was suppose to. Please help....thank you....