What software is driving this Firefox spell checker, Linux Mint 17.3, Firefox 47?
 

Solved:

Running Linux Mint 17.3, it's upgraded from Firefox 42.0 as installed, to version 44 to now 47.

I am called out for not calling a freeway a motorway, for speaking of color instead of colour. I have gone in to add-ons and disabled the overseas English laguage packs, added US English, removed the English language pack from my comuuter, thus removing the disabled language packs, cleared cache, restarted Firefox, logged out and back in and restarted Firefox. I failed to mention I also did a sudo apt-get update.

What do I get rid of next?

It's for the IBM computer listed in my signature block, I failed to add.

The Firefox spell checker appears to be driven by the language that you have selected from those available in firefox-locale-xx and any additionally installed add-on dictionary or dictionaries.

You can choose between the different languages available by right clicking on any input text, then selecting Languages->the language you prefer. From then on that language will be used for spell checking unless you choose another one using the same method.

I did right-click on this text while it was being composed and looked a bit further down the menu. "Languages!" All of these English languages are listed in a side menu and I selected for US. Freeway, motorway, color, colour. Last one is misspelled. It WORKS!

Thank you hydrurga. Now I've already done away all the language packs listed in Add-ons, so apparently I don't need them. I just needed to be reminded of how to change the language selection.
SOLVED

Excellent, although being British I think more people should be using non-US spelling anyway ;).

If you want to, you can mark the thread as "Solved" (see Thread Tools).

I just happen to like some of the British terminology and pronunciations. "Laboratory" comes to mind. Over here it rhymes with lavatory, and as a kid in school I remember wanting to visit the lab'ratory to relieve myself. The boys in the lab would not appreciate the sight! :)

Thank you for pointing me to the Thread tools. I will do just that.