That sounds like an add-on. Check that out.

I have no add-ons to do with spelling.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Type about:support in the address bar and press Enter. Under the main banner, press the button; Copy Text To Clipboard.. Now in the Reply Box on the forum page, do a right-click in the box and select Paste.

This will show us your system details. No Personal Information Is Collected.

Firefox doesn't automatically change what you have typed. Firefox only opens a context menu when you right-click on a word that is underlined and flagged as misspelled.

Do you have a dictionary installed and selected?

You can see which dictionary is selected if you right-click in a text area and open the Languages sub menu. At least one dictionary should be installed and have a check mark to indicate that it is selected.

• open the "Add Dictionaries" link to install a dictionary if you do not have one.

The spelling checker is enabled if [X] "Check Spelling" in the right-click context menu has a checkmark.

You can enable or disable spell checking globally:

• Options/Preferences -> General: Browsing: "Check my spelling as I type"

You can look here for dictionaries:

• https://addons.mozilla.org/firefox/language-tools/

You say it isn't Firefox and I'm also informed that, for example, Facebook isn't responsible. The fact remains that something is changing what I type.

I sense a buck passing around rapidly.

FredMcD said

. . . . . . . This will show us your system details.

PeterRobertson said

You say it isn't Firefox and I'm also informed that, for example, Facebook isn't responsible. The fact remains that something is changing what I type. I sense a buck passing around rapidly.

Have you checked the keyboard settings in Windows Settings  ?

Note that some website disable the builtin Firefox spelling checker and use their own spelling dictionary. Such websites might include auto-correction in addition of their dictionary. You might be able to disable their native spelling checker, but to enable the Firefox spelling dictionary you would have to remove the spellcheck="false" attribute or change its value to true by using a JavaScript bookmarklet or by editing this attribute. You can usually find this attribute if you right-click in the text editor area and open the Firefox Inspector.