Which web browsers are included in Trisquel version 8.0?

Abrowser is the default browser. It's a rebranded Firefox with better privacy settings, albeit not as restricting as Icecat would be. Ruben calls its settings "sane".

Thanks.
Is there a secondary web browser choice included too, or is Abrowser the only one?

Trisquel mini includes midori, but I strongly doesn't recommend to use it. Abrowser is the best choice in my opinion. If you are paranoid then use IceCat.

Remark: Trisquel's leader is Abrowser and IceCat's main developer.

Other than Abrowser or Midori: Netsurf, Epiphany (labeled 'Web', package epiphany-browser), Qupzilla, Konqueror, Icecat. Trisquel only includes free/libre software in its repositories.

There must be a reason why Seamonkey is not included, but it can be downloaded easily and run from its folder without installing.

Package 'torbrowser-launcher' helps download and run the Tor Browser Bundle.