That is currently not possible, you can only select one dictionary at the time.

• 1402822 - Spellchecker needs to allow multiple language dictionaries at the same time

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)

This seems odd since it lets me select multiple at the same time. It behaves exactly like https://support.mozilla.org/en-US/questions/785463 described when having multiple selected and trying to write Icelandic/Russian/English

Note that the bug I posted above has been fixed for Firefox 100 and newer.

• https://www.mozilla.org/en-US/firefox/100.0/releasenotes/

Firefox spell checking now checks spelling in multiple languages. To enable additional languages, select them in the text field’s context menu.

See also Switching dictionaries:

• https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker