The Options page, Applications list, doesn't allow adding entries in a free-form manner, and the underlying settings file (mimeTypes.rdf) is somewhat obtuse in its format, so it's probably going to be difficult to edit by hand.

Usually the applications themselves or their installers will restore their own registry settings, so you might need to re-run some of those or look up their help on how to restore those missing settings. After that, Firefox can discover those entries again.

For the SendTo application, do you mean the program or site that handles email links (or the Email Link feature)? Please see this article for more information on that one: Change the program used to open email links.

Thank you for your response. What happens is that if I click on an e-mail link on a web page, nothing happens. Likewise, if I click on File/Email Link, nothing happens. Trying to invoke those functions does not change anything on the Options/Applications lineup. I couldn't find anything on this particular issue in a lengthy search yesterday (before submitting my question). There were also many other program associations on the Applications list before than there are now.

This evening I did a complete restore of the registry as it existed before I ran the first scan of the registry, but this, alas, did not repopulate the Applications list. My next thought was a clean reinstall of Firefox, but I don't want to do that if there is any other option there, because I fear losing all of my options and settings in Firefox, which I have used since it replaced Netscape. If/when I get my Options unscrewed up, I will save them using the Firefox tool for saving them to my Firefox account online, which was recently introduced (at least to me).

Could you do this three-minute experiment to see whether a clean profile actually makes a difference:

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Click the Create a New Profile button. Assign a name like Aug2016, and skip the option to relocate the profile folder.

After creating the profile, scroll down to it and click the Set as default profile button below that profile, then scroll back up and click the Restart normally button.

Firefox should exit and then start up using the new profile folder, which will just look brand new.

How does the Options page Applications list look in the new profile?

If it looks good, I could advise on how to copy the fresh mimeTypes.rdf from the new profile folder to the old one. Otherwise, you can use the "delete the old file" method to generate a new one in the old profile.

Speaking of which, when you are done with the experiment, open the about:profiles page again, Set default to your regular profile, then Restart normally to get back to it.

I did as you advised above. The Applications under the new Profile name's options did not look much different than under my "real" profile (i.e., it was very sparsely populated), but, importantly, it did have the line to set or change the "mailto" setting.

I also went ahead and copied the mimeTypes.rdf file from the new profile, which had the mailto setting, into the settings under my "real" profile (renaming the one there as oldmimetypes.rdf), then restarted Firefox. The MailTo setting is now present under the Applications under my original profile.

Thank you very much for your help on this.

MoeCowbell said

I did as you advised above. The Applications under the new Profile name's options did not look much different than under my "real" profile (i.e., it was very sparsely populated), but, importantly, it did have the line to set or change the "mailto" setting.