I don't use preview enough to remember how it behaved before, but I see what you mean. Even though the script that updates the preview tab has this function call, which I assume is meant to switch tabs, it doesn't seem to work:

this.previewWindow.focus()

Firefox has a preference that generally prevents sites from changing the order of windows, for example, putting a popup in front of everything. If I disable that protection, then WordPress switches tabs. I don't know why it worked with this protection before but not now. If you want to try disabling that as a workaround while you're editing, here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste flip and pause while the list is filtered

(3) Double-click the dom.disable_window_flip preference to switch the value from true to false -- to change it back, you can just double-click again

If you wanted to get more to the bottom of this, and you have a free hours, you could run Mozregression, a tool to pinpoint the change that broke it. However, it would be helpful to have a much simpler way to test than having to keep logging into your admin area and editing a post.

https://mozilla.github.io/mozregression/

I filed a bug. Hopefully it will get fixed by Firefox 86 next month.

• https://bugzilla.mozilla.org/show_bug.cgi?id=1690099
• Please avoid adding "me too" or "hurry up" comments - https://bugzilla.mozilla.org/page.cgi?id=etiquette.html

Looks like this will get fixed in Firefox 86, expected to release on Feb. 23.

Thank you very much for your support!