seriously I can not find the question. But it sounds like menu people you have ignored or not been aware of the standard advice to compose your signature within Thunderbird, encountered a problem and then had to fix it.

About the worst thing you can do is paste from a word processing package. None really do a good job if it. Word is abysmal, including in Cascading Style Sheets as it does everything that is defined in the normal.dot file.

If pasting from such sources plain text Ctrl+Shift+V is preferable as it does not mess up the document your writing.

Matt, you are quite right that I, and probably many others who have contributed to this topic over several years, were not aware of the standard advice.

But even though I did not follow it then, the spell-checker has varied a number of times even when the signature did not. Up to last month, it had (correctly) ignored the invisible occurrence of "Panose" for a year or more. Suddenly, after the latest update, the so-called spelling error made its appearance again.

So how were the innocent (non-technical) users to realise that it was their own fault, not that of Thunderbird's experts?

More to the point, the detection is a bug and should be filed as one.

Unfortunately Thunderbird is at it's heart old and while some would call it mature one of the issues this brings is complexity. Small changes in one place cause regressions in others. This sounds like a regression we have had before as you say. So if you can reliably make it happen, file a bug. Give the developers the signature as a method to reproduce and they will usually fix such things fairly quickly as it is really annoying for everyone concerned.

File bug here https://bugzilla.mozilla.org/ Product: Core Component: Spelling Checker