If you install Firefox via repositories it might not install with your preferred language/settings. If your distro is in English it will default to American English as the main language and other English variations as additional spelling dictionaries.
You can change this from the CLI. The below examples will leave British English as the default buy it can be easily be adapted to your needs.
Be warned that the spellchecker is shared with other applications like LibreOffice.
Check which spellchecker is installed on your system. Older versions used myspell and current ones use hunspell.
$ apt list --installed | grep myspell
$ apt list --installed | grep hunspell
Older versions
# apt remove myspell-en-au myspell-en-us myspell-en-za myspell-en-ca
# apt install myspell-en-gb
Current versions
# apt remove hunspel-en-au hunspell-en-us hunspell-en-za hunspell-en-ca
# apt install hunspell-en-gb
