Border-Radius Causing Naughty Errors in FireBug: "Unknown property... Declaration dropped." Should I make this disappear or better keep it my way?

Open the drop down in your console tab and un-tick stuff like "show CSS errors".

Also, it's not a bad thing. If Firefox comes across a property it doesn't know (such as border-radius at the time this question was asked) it will just ignore it and continue with the next property. This is why for instance -webkit-border-radius: 2px; -moz-border-radius: 2px; border-radius: 2px; works. Firefox will ignore the -webkit- prefixed one, it would recognize the -moz- prefixed one and ignore the non-prefixed one because the non-prefixed one had not yet been implemented in the version of Firefox you used. (It is now no longer needed to prefix border-radius unless you're supporting an ancient browser)

You might want to pop the IE-specific properties (filter and zoom) into an IE-specific stylesheet, and include that with conditional comments.

As for the rest, you’ve just got an older version of Firefox that doesn’t recognise the newer properties. That’s fine, it won’t do any harm. (Somewhat odd that moz-opacity isn’t recognised, as I thought that had been around for ages, but it’s fine.)