What do you mean by share button?

Press <Alt>, the menu bar should show up. Select File and look for Email Link.

The share button may have been an add on or a social media api that is no longer used. However if you have add ons check the page "about:addons" to see if there are any that have disabled. Or if there is another profileUse multiple profiles in Thunderbird

If you reset Firefox then you should find a "Old Firefox Data" folder on the desktop.

See also:

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox