Disable Abrowser opening a Trisquel tab after update

The next time Abrowser is started after an upgrade it adds an annoying trisquel.info tab to open tabs.

This is useless and confusing to users. So, unless somebody wants to write a changelog I think this needs to be removed.

Apparently this can be disabled by going to about:config and then setting "browser.startup.homepage_override.mstone" to “ignore”.