If you go along the grey toolbar at the top of the Firefox browser, Evie, click on
Tools. This will operate a drop-down menu from which you should choose
Add-ons. The Add-ons box will open in a separate window. At the top left of that box you should be able to see an icon marked
Get Add-ons, which you should click.
This will list a variety of add-ons (also known as 'plug-ins'). Ignore these but look at the right side of the window and you will see a link marked
Browse All Add-ons, which you should click. This will open another tab in your browser. Down the left side of that tab is a range of add-ons you can choose. If you want a dictionary, click on the
Dictionaries & Language Packs button. This will take you to a list of available dictionaries. Scroll down until you find the 'English (US)' dictionary and click on the
Install Dictionary link on its right hand side.
That should install an English dictionary for you with American spelling variations. I'd like to talk you through to the completion of the job, but I'm not going to install it on my computer in case it clashes with my English (British) dictionary

.
Once you've installed the dictionary, whenever you type something into the browser which the dictionary doesn't recognise it will underline that word in red. If you then right click on the word, the browser will offer you a list of alternative spellings it thinks might be suitable.
Hope this helps
Gill
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There is no opinion, however absurd, which men will not readily embrace as soon as they can be brought to the conviction that it is readily adopted.
(Schopenhauer, Die Kunst Recht zu Behalten)