Option–Shift–Left Arrow in FireFox selects AND jumps to last

[JohnDohe]

Hi,

Up to the last 2 weeks or so, Option—Shift—Left Arrow would work to select whatever was
to the left of the cursor.

This is the default behavior for Option—Shift—Left Arrow in Macs:

https://support.apple.com/en-ie/HT201236

"Option—Shift—Left Arrow: Extend text selection to the beginning of the current word, then
to the beginning of the following word if pressed again."

Now executing Option—Shift—Left Arrow in FireFox 68.0.1 not only does the above noted selection,
but also causes FF to jump to the most recently opened/used tab !

This is not seen with Option—Shift—Right Arrow.

Additionally, repeating Option—Shift—Left Arrow takes me back to the original tab, showing my
selected/hi-lighted word.

Doing Option—Shift—Left Arrow again takes me back to the previous tab, and on and on, swapping
between the two tabs and hi-liting/selecting the next word to the left of the previous selection!

***This does NOT happen in Safari !!

In Safari, Option—Shift—Left Arrow does just what its supposed to do: Extend text selection to the
beginning of the current word, then to the beginning of the following word if pressed again.

Whats going on with FireFox?

Is this keystroke combination reserved for this function in FF ?

Or is this a new quirk with the latest release?

Thanks!