Thanks for replying. I think I found what I wanted at:
https://support.mozilla.org/en-US/kb/accessibility-services
Following the instructions I already had it enabled "0" so changed it
to "1".
In version 78 of Firefox there was an option in options to toggle this
feature off and on. The default was "on" in 78. My query mainly was why
it doesn't appear in the main options of the latest Firefox.
How do I disable Firefox Accessibility Service?
Type about:config in the address bar and press Enter.
A warning page may appear. Click Accept the Risk and Continue to go
to the about:config page.
In the about:config page, search for the preference
accessibility.force_disabled.
Observe the Value column of the accessibility.force_disabled row.
If it is set to 1 then do nothing.
If it is set to 0, double-click on it to open an editing field,
type 1, then click the blue check mark button or press the Enter key to
complete the update