How to install S Mime either in posteo via the browser or thunderbird

Hello

I want to install SMime in my email account (from posteo).

I prefer to use it via the browser instead of an emailprogram (thunderbird).

I tried to find an S Mime application in the package source. There are some, but after choosing and applying it it doesnt appear ...

So which S-Mime application would you choose and what are the next steps to install it either in the borwser as addon or in an emailprogram.

Thank you very much in forward!
Cathie