Electrolysis has been re-enabled in Nightly. It brings lots of end-user benefits but it also brings a new style for tabs. I guess this underlining is to help users easily notice the difference with a non-Electrolysis build.

But you can easily disable that:

1. Locate your profile
2. Go into the chrome directory. You may need to create it if it doesn't exist.
3. Create a userChrome.css file with the following content:

   .tabbrowser-tab[remote] {
     text-decoration: none !important;
   }

4. Restart your Nightly
5. Enjoy!

Thanks Jonathan Kew for the tip.