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

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.