see a helper application in preferences for HTTP and HTTPS document types.

More details here https://support.mozilla.org/en-US/kb/hyperlinks-in-messages-not-working

Hi Matt, and thanks. I managed to follow the information on "Make Firefox your default browser" in order to change the default browser for a short time only which is what I needed. The information though on changing the default from within Thunderbird didn't work in my version. In my version of Thunderbird (78.14.0 64 bit) I couldn't go into Settings, General, Startup, etc. My Thunderbird only has Account Settings - not operational settings. In my case I needed to download a music album. It was in zip file format and it wouldn't download in Firefox. I had to do it in Chrome. It would be really good if you could just change the default browser easily from the within the program if you want to just open a particular link in another browser.

options was renamed preferences in V91 just released.

the revised config editor article might help and I have updated the article on hyperlinks for the new menu locations in V91. So you might want to look again and tell me if it actually makes sense.

Well thank you Matt. I hope I never have to use the Config Editor. The Hyperlinks article is also very clear. I just hope I never have to use it either! One question, why can't they just do a simple thing like enabling you to right click a link to temporarily choose another browser?

I have been playing this game of support for most of the last decade. I have seen perhaps 3 people over those years that wanted a different browser set for Thunderbird. Even less looking for a right click open with as you would find in windows file explorer. If you are one of two or three in a group of millions your desire is unlikely to see the light of day simply because no one is asking for it.

"Game of support"? Well you must enjoy doing it. Have a nice day.