[Affected versions]: - DevEdition 60.0b7 - Nightly 61.0a1 - Firefox 59.0.2 [Affected platforms]: - Windows 7 x32 - Ubuntu 16.04 x64 - macOS 10.13 / Mac OS 10.11 [Steps to reproduce]: 1. Go to about:addons (Get Add-ons) 2. Activate one of the featured themes from the page. 3. In another tab sign in to Sync 4. Go back to about:addons (Themes) and observe behaviour. [Expected result]: I am not sure what is the expected result here. It's either: - The old theme is kept and the new theme appears in about:addons (Themes) or: - The new theme is kept and the old theme can be found in about:addons (Themes) [Actual result]: - the theme appears in about:addons (Themes) as enabled and you can't disable it. - visually the theme changes to the one you had before on your Mozilla profile and if the page is refreshed, the theme from step 2 disappears from the list. - in about:addons (Get Add-ons) the theme selected in step 2 remains activated and if you try to deactivate it an error is displayed: "An unexpected error occurred during uninstallation." When the page is refreshed, the issue disappears. [Regression range]: - I am not sure if this is a regression, but if I find one, I will put it in a comment. [Additional notes]: - When the sync profile doesn't have a theme selected, the new theme is listed in the about:addons (Themes) and can't be disabled. And in about:addons (Get Add-ons) the theme looks like it's activated and if you try to deactivate it an error is displayed: "An unexpected error occurred during uninstallation." When the page is refreshed, the issue disappears.

I don't think this is a regression. I could reproduce this issue even on Nightly 31.0a1 (2014-04-26).

Realizing this isn't a straight dupe, but if this is about LWT, it's moot with the migration to static themes.