There isn't an interactive way to edit the Applications (download handler) list. mailto really should be there by default. If no one has a better suggestion, you could rename the relevant settings file and have Firefox rebuild it. Here's how:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in File Explorer.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > Exit/Quit
• (menu bar) File > Exit (or Firefox > Quit)

Pause while Firefox finishes its cleanup, then rename handlers.json to something like handlersOLD.json. If you see a file named mimeTypes.rdf, rename that one to mimeTypesOLD.rdf.

Start Firefox back up again. Is mailto back?

Thank you for the response. I tried this myself while waiting for someone to answer my post, after doing a bit of research.

This works, but it gets rid of most of the existing entries in the applications list. Now the mailto entry is there, but many others are not. E.g., XLS and ZIP.

What causes an application to get added to that list?

Files like XLS and ZIP should be added after you download one and choose a custom setting. Otherwise, I believe the options are set up dynamically with a fresh registry read each session.

The handlers.json file is a plain text file in JSON format which is editable, but it isn't easy to read. If you want to experiment you could carefully paste in the new entry for mailto into your old file and see whether Firefox accepts it. Possibly more trouble than it's worth...

Thanks. I tried that. :) I pasted the mailto section into the old version of the json file. It worked, in the sense that there is now a mailto entry. But the choices in the dropdown menu for that entry are not what they should be. There is no option such as Use Yahoo or Use GMail.

Anyway, I think I am just going to leave it as is with the defaults and then worry about it if something else pops up to indicate a problem. At least the mail icon now works to send links.

Thanks for your help!

You mean this here? Did you try the other to see if anything else pops up there?

Try these links to see if they will addin

https://compose.mail.yahoo.com/?To=%s

https://mail.google.com/mail/?extsrc=mailto&url=%s

Don't know if they will work but that what I get when I look at the links for mailto options.

Thanks! I get a server error with both of those links.

I am considering this issue solved, however. By renaming the handlers.json file I was able to get Firefox to recreate the default.

WestEnd said

You mean this here? Did you try the other to see if anything else pops up there? Try these links to see if they will addin https://compose.mail.yahoo.com/?To=%s https://mail.google.com/mail/?extsrc=mailto&url=%s Don't know if they will work but that what I get when I look at the links for mailto options.

I see this mailto section in a handlers.json file.

,"mailto":{"action":4,"handlers":[null,{"name":"Yahoo! Mail","uriTemplate":"https://compose.mail.yahoo.com/?To=%s"},{"name":"Gmail","uriTemplate":"https://mail.google.com/mail/?extsrc=mailto&url=%s"}]}

Note that you can find the uriTemplate on the about:config page.

• gecko.handlerService.schemes.mailto