jscher2000
Another option would be to run a little snippet of script in the Web Console (Ctrl+Shift+K, or on Mac, it's Command+Alt+K) to chop off the last character of the title:
document.title = document.title.slice(0, document.title.length-1);
If there is still something strange on there, press the up arrow and submit it again (or if there's a Run button, click Run again).
I don't know whether any errors would be recorded when Firefox tries and fails to generate the list. While viewing that window, you could open the Browser Console popup window (Ctrl+Shift+J), then click the List All Tabs button in the main window and see whether any error messages appear in the Browser Console in response.
Why would you want to do that?
Probably a habit learned in a Chromium-based browser.
Here's an example script to reassign Ctrl+Shift+B from back in the day when its function changed: https://www.reddit.com/r/firefox/comments/kilmm2/restore_ctrlshiftb_library_by_setting_configjs/
Hopefully that gives you a framework, and I have a more detailed reference article here using a different example (menu hotkey change, not keyboard shortcut): https://www.userchrome.org/what-is-userchrome-js.html#combinedloader
They are part of the built-in search URL for Google:
I don't know why channel shows as "fenc", though; I have no idea where that is coming from.
The Archive.org Wayback Machine has a copy at https://web.archive.org/web/20230314134254if_/https://addons.mozilla.org/firefox/downloads/file/4038541/onenote_clipper-3.9.0.xpi
I haven't tried to load it to see if it will verify (i.e., whether it's a perfect copy).
According to a cached copy of the old page, it was released under the MIT license, which would make it free to modify and redistribute subject to some minor notice requirements. So it can live on if someone wants to move it forward. (There seem to be a lot of repositories on Github, but perhaps not specific to Firefox?)
Not sure of your OS but Windows 10 has a global setting to "Underline access keys when available" that I think makes them always visible.
If you need to remove this access key, one option is a startup script. In my example in the following article I refer to them as accelerator keys:
I wasn't able to replicate the error by adding one or two m or n dashes to the document title. Based on this error message and the screenshot, and comparing how Firefox displays other tab titles when Multi-Account Containers is installed, I wonder if the problem tab was open in a container that had been deleted or had become corrupted in some manner so Firefox had retained the assigned container ID, but that container ID no longer matched any container. Opening the URL in a new tab most likely used a different container, working around the problem.
UPDATE
I filed a bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1848877