Every time I've installed firefox on a new laptop, I've done the same silly things:
* I've generally gone a few weeks before realising that spellchecking is not happening. (Missing red underlines, but I'll always be looking for spell check controls with a right-click in a textarea too)
* I've found the bit in "settings" to enable "check your spelling as you type", but it's already ticked. Confusing!
* I've googled and found myself on this page, which has the feel of an obscure forum posting: https://support.mozilla.org/bm/questions/1337289 but has all the info I need.
* In a fit of over-confidence (and bad memory of fixing it the last time), I've hastily followed the link to language tool addons https://addons.mozilla.org/firefox/language-tools/ and then gone for the top link "English (GB) Language Pack", a few confused minutes later I realise: no! it's a "dictionary" I need.
THE IDEA: How could a freshly installed firefox make this more obvious? I can think of a few ways:
* It could prompt to install dictionaries as part of initial set-up (But ok. Maybe you're working to keep that journey short)
* It could have a right-click context menu option "Set-up spell checking" or something, when spell checking is not happening.
* In settings, the "check your spelling as you type" option could be greyed out with a link to dictionary downloads if there's no dictionary installed surely? (No use being ticked if there's no dictionary, right?)