Support TB 91

[devmount]

Is your feature request related to a problem? Please describe.
According to Mozillas Release Calendar, TB 91 will be released in August 2021. As ThirdStats uses the current WebExtension APIs, it's already compatible with the current TB daily releases. However, there are still some issues to address to make ThirdStats not only run smooth on TB 91, but to also take advantage of new features and the new multi-thread-architecture. It looks like TB 78 will be supported until October 2021. I will try to keep ThirdStats compatible with TB 78 as long as possible. When the daily user stats show that a significant amount of users upgraded to TB 91, I will drop support for TB 78. I expect this to be in Q1 2022, but will announce it properly when that time has come.

Describe the solution you'd like
The following issues should be addressed:

• Fix Favicon in Thunderbird 91 #333
• fix visual appearance of fonts and icons
• fix visual appearance of form elements (e.g. color of select drop-down content)
• show menu popup in configured color scheme (currently it's always dark)

Additional context
There surely are more issues, so anyone who has time to test the daily or beta builds of Thunderbird with ThirdStats is a great help!

[devmount]

Implemented with #332