Best practice: browser version and mobile-friendly

Hi,

I’d like your opinion/expertise on this one. What would you think if I ran a check whether the visitor uses Firefox and/or a mobile device and print a message?

My concerns: a) My site looks really bad in latest Firefox (strips, fonts, elements’ placement, etc), b) My site, being heavily menu/options driven as a big database, is very difficult to be properly displayed on mobile phones.

Along your thoughts, code-wise, where should I look at for these checks?

Thank you.