solution: Thunderbird "already running" error on mailto link

[dlbert_g]

I am not a programmer. I wanted to report and highlight a problem I solved.

If I had Thunderbird running, clicking on a mailto link in FF or clicking Send Link in the menu (a.k.a. Send Page as Link) would fail. I would get the infamous error that Thunderbird is "already running", please close etc.

For a while, the problem was also repeatable in the opposite direction, starting a Compose window and then trying to launch TB main interface. I thought this was solely due to errors I induced by moving my profile from C: to D: ---- and it was mostly that, but I got dismayed when I could not fix it by editing files (or About:Config in TB), replacing strings pointing to my old profile. I created a new profile and dragged my mail (but not extensions) into it, then moving various files for folder layout, etc.

Eventually I was certain I had all the .DATs and .RDFs and prefs.js etc. fixed in my new profile, but it still didn't work from Send Link or Mailto in FF.

I also checked the Registry for Mailto, but it looked correct:
"C:\Program Files\Mozilla Thunderbird\thunderbird.exe" -osint -compose "%1"

That's when I started wondering if it was Firefox itself, and then I thought of NoScript, since I know NoScript purposely blocks XSS, and Mailto is kinda like an XSS. First, I outright disabled NoScript. Send Link and Mailto then worked. I re-enabled NoScript and it was broken again, as I expected (which was gratifying).

Here's what I found:

I enabled Local Links permissions under Trusted Sites. I also enabled Allow Rich Text while I was making changes. (I'm not clear of what the drawbacks could be, or why these are normally unchecked, except the slight risk of a locally stored HTML or MHTML file with a mal script. More?).

I also added 127.0.0.1 to my NoScript Whitelist. I may have had that there previously and removed it out of ignorance.

I'm not at this point certain which of the three changes fixed my problem. Still, clicking on a Mailto, Firefox opens a blank Tab which remains Untitled, but I can live with closing that Tab, no prob. I just did not want to have to launch a Compose window then copy-and-paste URLs from FF. Definitely a petty annoyance.

Next task, Peace on Earth. Wish me luck.

Since I found it impossible to find this specific answer on the web --- plenty of solutions to that error due to moving TB profiles, where TB would not start at all, but nothing on the specific problem of Mailto trying to launch a 2nd instance of the TB app --- I wanted to post this here for others to find.