Yep, just got a call from a bunch of my customers about this. You used to be able to manually set it in 'about:config' but that functionality no longer exists.

Hope this is a bug and not removed functionality.

UPDATE: This setting is no longer available in Firefox 119, unfortunately. A less convenient workaround can be found in: https://support.mozilla.org/en-US/questions/1429073#answer-1614190

You can restore the orientation selector by making the following setting change. This overrides the ability of a web page to specify a paper size, which Firefox interprets as locking the orientation.

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

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future. Since we have had this setting for a long time and I use it myself, I feel comfortable mentioning it.

(2) In the search box in the page, type or paste layout.css.page-size.enabled and pause while the list is filtered

(3) Double-click the preference to switch the value from true to false

that did the trick... thank you!