firefox are installed as extensions. You should be able to remove any you don't want by looking through the list of installed dictionaries in "Tools"=>"Add-ons"=>"Extensions".
Alternatively, you may be able to delete them directly from the extensions folder. In my case, the three non-English dictionaries I have installed all have a folder called
firstname.lastname@example.org. You can have a look at the ones you have installed with this command:
find ~/.mozilla/firefox/*default/extensions/ -maxdepth 1 -name "*dictionaries*"
Or, if they are installed system-wide:
find /usr/lib/firefox-addons/extensions/ /opt/firefox/extensions/ \
-maxdepth 1 -name "*dictionaries*"
Once you have located them, you should be able to remove the dictionary entry by simply deleting the folders in question. For example:
find ~/.mozilla/firefox/*default/extensions/ -maxdepth 1 \
-name "*dictionaries*" -delete