Error: “Firefox Doesn’t Know How to Open This Address” — But Email Program Is Specified

I was using Firefox ESR 24.4.0. I clicked on a mailto link. That is, the webpage I was reading displayed an email address. I moused over that address and verified that, at the bottom left corner of the screen, a little box appeared, saying, “” (replacing “xyz” with the actual address). I clicked on the email address. Firefox opened a tab with this message:

The address wasn’t understood

Firefox doesn’t know how to open this address, because the protocol (mail) isn’t associated with any program.

You might need to install other software to open this address.

I ran a search and saw that virtually no one was reporting that exact error. I decided to commence this post to resolve it.

I should not have been getting the error. I had clicked on mailto links many times, in many webpages, and it had worked: my default email program (Thunderbird) would open up and present me with a blank email, addressed to the mailto address. Why had it not worked in this case?

I checked Firefox Tools > Options > Applications > Content Type: mailto. It did indicate that Thunderbird was correctly set to be my default email program. Thunderbird was up and running normally. So that didn’t seem to be a source of the problem.

One solution, I thought, might be to update my version of Firefox. I clicked Help > About Firefox. It developed that there was an update. So I ran that and prepared to restart as suggested.

At this point, however, I double-checked the mailto link in the webpage I was viewing. On closer examination, the tiny box that popped up at the bottom left corner of the screen did not show what I represented above. Instead, it showed “mail: to” That is, somehow the webpage designer had screwed up the mailto link. I tried clicking on a properly designed mailto link on another webpage to double-check. Sure enough, Thunderbird opened up a blank email as expected. Problem solved!

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