You appear to have a lot of dictionaries installed in your Linux distribution. You can possibly remove all unneeded dictionaries and leave only the ones you use.

You can check these locations for globally installed dictionaries:

/usr/share/hunspell/
/usr/share/myspell/

See also the DICPATH environment variable. Some of these dictionaries can have a underscore (en_US) instead of a dash (en-US) in the language selector.

Are you using a package build of Firefox from your Linux distro or such? They tend to also install packages for disctionaries and language packs for Firefox in the package manager you use.

Normally the official builds of desktop Firefox (for Windows, macOS, and Linux) from www.mozilla.org/firefox/all/ only comes with one dictionary or no dictionary depending on the locale due to license reasons. In this case you can install dictionaries from https://addons.mozilla.org/firefox/language-tools/

Thank you @cor-el. `sudo apt remove hunspell-{ar,be,bg,bs,ca,da,de-at,de-ch,el,eu,fr-classical,gl,gu,hi,hr,hu,id,is,kk,kmr,ko,lt,lv,ml,ne,nl,pl,pt*,ro,ru,si,sl,sr,sv,sv-se,te,th,vi,cs} myspell-{eo,et,fa,ga,he,nb,nn,sk,sq} aspell-{a*,b*,c*,da,el,eo,et,eu,fa,g*,h*,i*,k*,l*,m*,n*,p*,r*,s*,t*}` did improve the situation and I get you don't see this as something that should be fixed in FF but I still see a full screen height of mostly spanish locales that I guess are part of some all-locale spanish package.

Regardless of me maybe getting this acceptable this way, I do believe it should be on firefox to facilitate the switching between dictionaries.