Firefox still supports mobile plugins but, a while ago (IIRC last year), the plugin API changed, as a result currently the selection of available plugins is much smaller than it used to be. I should add that I haven't checked in a few months, maybe it's improved now.
They didn't change the plugin API, they did a massive rewrite of the mobile browser engine infrastructure that broke most of the plugin APIs, and probably the test suites as well. So they are disabled until they can be fixed and proven to work.
