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.