There is the Spell Checker, and there is the Dictionary. These use separate data files. When you add a word to the Spell Checker, it is not added to the Dictionary.

There may be a program or add-on that does both, but I don't know.

A possible misconception for some may be that the option only checks (and not corrects) the spelling, so that should be normal. Recognized words are ones in either one or more installed dictionaries OR the one containing manually added words, and should just no longer be highlighted after adding them manually.

However, you may be right - added words do not show up in suggestions when typed wrong another time afterwards. There is actually an old but open bug about this. More info can be found in this mozillaZine KB article.

Perhaps the Grammarly for Firefox or similar add-on may offer what you need. I haven’t tested it, but it may offer the feature to implement.

Thanks, both. I have Grammarly and I did not think about using like that I will check it, the only problem is that I write mostly in Spanish and I want that too.

You can use one, or the other. You can't have both at the same time.

Suggestion: Create a second profile. One English, the other Spanish.

Remember that when using 2 or more installed dictionaries in Firefox, the selected one should be remembered per website and even after a Firefox restart, so there may be no need to use separate profiles. If you want both locales to be checked for the same website, you can probably open it for another locale in a separate container tab, but I haven’t tested that.

For the bug about no displayed suggestions from words added manually, you can vote for it or add a comment when logged in to Bugzilla, though I wonder if that will speed things up. Please use comments only when adding valuable info in order to get them fixed, no "I suffer this toos" - votes will of course do no harm.

Feel free to select one of the replies as a solution if you consider it as such.