Hi Caragen, In the What's New for version 69 a change was made:

See what’s new in Firefox! https://www.mozilla.org/en-US/firefox/69.0/releasenotes/?utm_source=firefox-browser&utm_medium=firefox-browser&utm_campaign=whatsnew

"Firefox no longer loads userChrome.css or userContent.css by default improving start-up performance. Users who wish to customize Firefox by using these files can set the toolkit.legacyUserProfileCustomizations.stylesheets preference to true to restore this ability."

Note that if you had updated to Firefox 68, this pref would have been set automatically to prepare for Firefox 69 where this pref will default to false (i.e userChrome.css and userContent.css are disabled by default). If you create a new profile or refresh Firefox then the same would happen.

See also:

• https://www.userchrome.org/firefox-changes-userchrome-css.html

Thank you. Cor-el...that explained why one PC kept the Tab on Bottom even when it went to 69. But all my other PC's were stopped at 67 while I waited. And thank you, too My_Cheese_is_Slippin.

But just as an Aside-- Really... what is the rationale behind Tabs on Top.

It is because someone upstairs thinks it's really cool? And How DARE these philistines put their grubby little fingers on my design imperatives?

Because it always seems like someone is repeatedly 'Taking Candy away' from what they think of as unruly children.

In current Firefox releases the Tab bar (#TabsToolbar) is in a separate container (#titlebar) together with the Menu bar (#toolbar-menubar) which makes it more difficult to get the Tab bar in the bottom position (i.e. requires absolute positioning for the Tab bar in the #navigator-toolbox) and CSS code for userChrome.css easily gets broken.

Caragen said

... Really... what is the rationale behind Tabs on Top.

Tabs on Top has been the default for over 8 years now. Since Firefox 4.0 was released in March 2011 each user has needed to 'toggle' a preference in Firefox (until Firefox 29 when that pref was removed); install an add-on / extension (until Australis when those extensions disappeared); or employed a userChrome.css file.

As far as "unruly children" - your choice of words; mine would be a bit different.

https://en.wikipedia.org/wiki/Red_Forman - Red Forman had a term for that as it related to his son Eric ....

Note that there is another change in Firefox 71 that requires to add a "display: block rule;" to the #TabsToolbar container.

• https://www.reddit.com/r/FirefoxCSS/comments/da72wv/psa_for_nightly_users_potential_breakage_or_even/