Hi you do not say if your theme is done by code you used in userChrome or if it is a Persona Theme or a Theme from https://addons.mozilla.org/en-US/firefox/themes/ . I would suggest you maybe look up the theme if is from add-ons and see what the support and info on the page says and also contact them if info is there.

If it is userChrome some code changes in version 60 made some code not able to work or partially broken. You would have to look for updated code for after ver 60.

Thanks for the prompt reply @Pkshadow. As mentioned it is a theme that is applied from the Add-ons Manager. Unfortunately the problem is not just related to a specific/single theme, it is any theme that is applied.

Todd-M said

Just in the last few days, my theme seemed to no longer be applied. If I go into the Add-ons Manager the theme is still selected and enabled, if I disable and re-enable it is applied, but only until I open go to another tab then it is overridden again by the default?

Sometimes a version of a Extension installed in Firefox could cause a interaction issue.

The Themes is this case are just simple background images. The Real proper themes are actually https://addons.mozilla.org/firefox/complete-themes/ which can take a lot of work and time to make and maintain by the authors depending on completely and quality of theme base used to work with. However since Firefox 57.0 and later they are no longer usable.

Pkshadow said

Hi you do not say if your theme is done by code you used in userChrome or if it is a Persona Theme or a Theme from https://addons.mozilla.org/en-US/firefox/themes/ . I would suggest you maybe look up the theme if is from add-ons and see what the support and info on the page says and also contact them if info is there.

The "Personas themes" now known as "lightweight themes" which is on https://addons.mozilla.org/firefox/themes/ are basically just background images that can be created very quickly. https://developer.mozilla.org/en-US/Add-ons/Themes/Create_Your_Own_Firefox_Background_Theme

Persona without the s for Mozilla used to actually refer to https://en.wikipedia.org/wiki/Mozilla_Persona which has been replaced by Auth0 https://auth0.com/blog/auth0-mozilla-partnership/

Thanks James, the problem is it is not an issue that is specific to an individual theme, as far as I can tell any theme I apply (whether it is from https://addons.mozilla.org/en-US/firefox/complete-themes/ or https://addons.mozilla.org/en-US/firefox/themes/) has the same behaviour, as soon as I open a new tab the theme is overridden by the default scheme on all open tabs.

Think this will be fixed when the following bug is fixed: https://bugzilla.mozilla.org/show_bug.cgi?id=1415267