Tested with:

Firefox 100.0a1
Firefox 99.0b8
Firefox 98.0.2

Tested on:

Windows 10 x64
Ubuntu 20.04 x64

Steps to reproduce

1. Open Firefox.
2. Login to https://www.facebook.com/
3. Click on "What's on your mind" box and then from "Creat post" popup right click on the text area and from the context menu select Languages and make sure "English" is checked.
4. Introduce any misspelled text in English.
5. Right click on any of the misspelled texts and select a word from the list of suggestions.
6. Right click again and click undo.
7. Right click again and click redo.
8. Repeat steps 7 and 8 randomely.

Expected result

The undo/redo options should work accordingly after using the Check spelling.

Actual Result

The undo/redo options are not working properly after using the Check spelling.

Note

This issue is not reproducible on reddit.com and www.onlinetexteditor.com tested so far. Could be Facebook specific issue.