Hi,
just wanted to leave this here for the googlers:
Not just websites, but also some addons like Stylus for Firefox store their data in the “Firefox HTML5 storage”.
So do not delete Firefox HTML5 storage if you use Stylus or in this case all your styles will be lost. It may be the same issue with the similar addon Stylish.
I wasn’t aware of that before… 🙁
Greets, Jerry
Firefox 54.0.1 (32-Bit)
Stylus 1.1.0 (2017-06-15)
CCleaner 5.32.6129 (64-bit)
CCEnhancer 4.4.2 (2017-07-14)