AdBlock is available for MicroB http://maemo.org/packages/view/adblock-plus-1.0/, didn't find noscript but it should be doable as well.

I guess any security issues in the Gecko rendering engine (these are the ones that would be shared with Firefox proper) will be fixed on Maemo as well as soon as the fix is available upstream.

Firefox mobile aka. Fennec also shares little more than the rendering engine with firefox proper (I'd guess the XUL stuff too but that's about it) so vulnerabilities in firefox proper are not automatically vulnerabilities in fennec.