I don't see why not, -but why serve content conditionally to a compliant browser, when 'compliancy' should be aboveboard and a conditional statement is present to serve the segregate non-compliant browsers (opps, -browser), and feed them the handicapped rules.
At any rate, -I have never seen this used yet And I wonder if "<!--[if firefox]>" would even be recognized. I beleive that the Mozilla group refer to Firefox as "Fx" (Capital "F" and lower-case "x", not "FF" as so many people here abbreviate it).
I have never seen it, and couldn't imagine why you would ever want to use it anyway. Let's face it, the only browser you ever need to specifically target is IE ....
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