What should be implemented/improved?
To achieve filtering out log messages containing specific keywords the search filter should have an option to invert the search result.
Test case (borrowed from issue 4368):
1. Open Firebug at http://wtest.migvan.co.il/art/fb-test.htm
2. Enable and switch to the Console panel
3. Type "g66 is not defined" into the search field
=> Just logs containing this phrase will be shown
4. Click "Invert results" in the search options panel (not existing yet)
=> All logs, which do not contain the phrase "g66 is not defined" will be shown.