We had enabled compression on our flight simulator forum but we are getting errors on fire bug(extention for firefox). The error is browser not defined. Because of this reason we had to turn java compression on in our site. But we have css compression working smoothly. If you could explain why this happens, it would be of great help. We can save atleast 100 kb with compression enabled.

Do you mean Java or JavaScript?
Did you guys turn compression on or off because of the issue?
When you say compression do you mean it is minified?

If you could explain why this happens

The compression is causing some error in the code, probably by stripping out some "white space"