Abrowser is Trisquel's default browser. Install Trisquel! :-)
That said you can access Abrowser's plugin catalog here: https://trisquel.info/en/browser/addons
And here is IceCat's: https://directory.fsf.org/wiki/IceCat (second half of the page).
Those add-ons can be installed in any Firefox-based browser. You can keep on using http://addons.mozilla.org too... but you then need to check the license by yourself.
