Linux: Kernel 3.4.106 compiled from source, optimized for my system Firefox is version 39.0.3. Installed 38.2 over 31.7 untared the source into a directory cp -Rf thunderbird/* /usr/lib/thunderbird Checked the symbolic link of thunderbird to be certain it pointed to the executable in /usr/lib/thunderbird. Hyperlinks in email no longer open a tab in firefox. Tried running in safe mode, same result. Tried resetting all the network.protocol-handler attributes to true; same result. Repeated with a clean install of 38.2, same results. Reinstalled 31.7. I had tar'd the original install prior to the update, something I normally do. I removed all the files in the thunderbird folder and replaced them with the previous install of 31.7. All now works correctly. I will try to update thunderbird on my slackware system and see if the problem also exists there.