Very interesting but kinda disturbing ..... I clicked on a email address of a company at the bottom of their web page to ask them a question and I got this pop up notice ..... I don't use Thunderbird Mail and don't have it downloaded as far as I can find .....
I'm now using Brave Browser but I can't find any reference to them having an email service ..... I know when I was using Vivaldi they would always try to get me use their service .....
DUH ...... never mind I see that must be the companies preferred email service ..... is it ok to use that one or better to use one of my own ?????
Not sure if this helps I had a similar thing with chrome? Email links?
On Windows, it will often open with Outlook.
Linux defers to Thunderbird instead of Outlook.
It's just a default mail provider and you can change it in your System Application Defaults settings.
But if you are not using a mail managing app, I wouldn't bother. I never use it; I just copy and paste the email address into my Protonmail in the Web browser and never get that Thunderbird Popup.
Sometimes I have to contact webpages or software owners so I search for the e-mail address but sometimes it doesn't open Gmail soon or it's not easy to see or find, some pages are an ocean of buttons. Usually I can just point to the hyperlink and see the address, otherwise I view the source code with the Firefox console and I find the e-mail address, for me this is enough as workaround.
The simplest thing is to right-click and copy the hyperlink rather than left-clicking. You can get yourself a free @e.email address and 1 Gb of cloud storage (they use Next Cloud client software).
In terms of clients there are only two that I would use, Clawsmail (formerly Sylpheed), and Evolution. Thunderbird is controlled too much by Google.