In fact, the situation appears to be more complicated, and points to a race condition of some sort.
Having fixed the problem, I thought I'd try a theory about what causes it. So, I removed the dictionary and restarted Firefox. However, my spelling language was still correctly set to en-GB.
Further, I can no longer reproduce the problem in a fresh profile, which of course does not have the dictionary installed.
I should stress that the only thing I changed in any of this was my Firefox profile; I did not add, remove or otherwise alter any Ubuntu package.
Hence, I really can't see what could have fixed the problem (since it has gone from being reproducible with a fresh profile to not being reproducible with a fresh profile), and I wonder whether there may be a race condition; not least, because the code that scans for dictionaries runs asynchronously.
In fact, the situation appears to be more complicated, and points to a race condition of some sort.
Having fixed the problem, I thought I'd try a theory about what causes it. So, I removed the dictionary and restarted Firefox. However, my spelling language was still correctly set to en-GB.
Further, I can no longer reproduce the problem in a fresh profile, which of course does not have the dictionary installed.
I should stress that the only thing I changed in any of this was my Firefox profile; I did not add, remove or otherwise alter any Ubuntu package.
Hence, I really can't see what could have fixed the problem (since it has gone from being reproducible with a fresh profile to not being reproducible with a fresh profile), and I wonder whether there may be a race condition; not least, because the code that scans for dictionaries runs asynchronously.