I have requirement to set my UI in both version but there are some CSS problem in that so, i'm looking for css hack so that I can define different css to both the version.
Please do needful.
I don't think there is a specific, pure CSS hack for targetting a specific version of Firefox. Not like with IE where you have the star hack and the underscore hack etc. And there is nothing like conditional comments either.
The only way to do something remotely like it is to find a CSS selector that is only supported by the version you want to target. A few years ago it was possible to target Safari using nth-child selectors because it was the only browser that supported them, but these days most browsers support this selector.
The problem with this type of targetting is that all (decent) browsers will eventually support it, so the hack will not be future-proof. And aside from that, I don't think there are any selectors that 3.6.11 supports which 3.6.3 doesn't...