If links are not opening up in the browser, try change your default browser to another browser and then change it back. In some scenarios after a browser update, the OS setting for the default browser fails to get updated.

For more information, check out the article: Hyperlinks in Messages Not Working

Thanks Chris (my hero!)

I tried all the usual fixes recommended here: https://support.mozilla.org/en-US/kb/Hyperlinks-in-Messages-Not-Working

Then, in accordance with this solution, I changed my browser to Google Chrome and back to Firefox. I still can't open links in emails. Thunderbird is simply unresponsive when I click or double-click a link.

Hi Bill,
You should use https://support.mozilla.org/en-US/questions/new where volunteers can get more details about your setup.

Because more people might start using it for their own questions, I'm going to lock it.