I think I saw another Mac user reporting this. Is the problem with

• command + →
• option + →
• command + shift + →
• option + shift + →
• something else?

Seems like switching tabs should not occur if you are in a form field, but maybe Firefox 127 has lost track of that for some reason.

Does the problem occur on basic form fields (like on this site), or on rich text forms, like email editors?

Oh wait, do you use the Tree Style Tabs extension?

https://connect.mozilla.org/t5/discussions/option-shift-arrow-now-switches-tabs-since-v-127/td-p/59298

Thanks for the replies. I haven't historically myself added this extension, but I'm going to check them and clean them up to try to approach it that way. This has been annoying for at least several days but perhaps a couple of weeks, I can't really track timing too well at this point :)

Option–Shift–Right Arrow: Extend text selection to the end of the current word, then to the end of the following word if pressed again

This is what it USED to do... now I briefly see the word highlighted before jumping far and away to the first tab ever opened... mind-boggling and vertiginous ;)

UPDATE: YES, that tree-style tab extension was in fact loaded. I don't recall it nor understand it nor care, it has never been interacted with and I don't see a need for it so I deleted it. Clearly SINCE the last update, that extension is (perhaps "again") causing issues. THANK YOU!