This web-extension will allow the built-in spellcheck feature of Firefox to be activated in every text-field, in every-page you visit, regardless if that website allows (or god forbid) blocked the spellcheck feature in that text-field.
You already have a dictionary, it is native to the language of your browser.
You can replace it or install an additional dictionary, following instructions in: https://www.ghacks.net/2014/04/30/add-languages-google-chromes-spell-checking-functionality/
There is another web-extension (named 'AutoComplete-Enabler') that acts similar way,
allowing the browser's built-in autocomplete-feature on every text-field, and it synced with your profile as-well. It may save you a lot of typing...
<strong>Additional Note, For Chrome Users:</strong>
There is a way you can make the <strong>spellcheck</strong> process run in the background,
which is useful on large-amount of texts.
Either change: <code>chrome://flags/#enable-idle-time-spell-checking</code> (<code>Enable idle time spell checker</code>) to <code>enabled</code>,
or add to the command-line (just after your <code>chrome.exe</code>) <code>--enable-blink-features="IdleTimeSpellChecking" --enable-features="IdleTimeSpellChecking"</code>.
I've basically made this web-extension for myself, and then decided to share it with everyone, because sharing is caring. it's nothing fancy but it is small, quick and it works. If you've enjoyed using it I'll be thrilled to hear all about it in the review section.
SpellCheck Enabler is 100% free (as beer..), include no ads (I hate those!), does NOT collect any data, include NO analytics and works entirely offline.