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In new versions you can simply click on the bug icon in the code navigator and choose 'Text size'.

OLD Version
1.Right click on the bug icon in the status bar.
2.Select "Open Firebug in New Window"
3.Select "View" > "Text Size" > "Increase Text Size"


The changes should stick if you revert back to opening Firebug in the same window.

Source: http://stackoverflow.com/questions/260008/increase-the-font-size-in-firebug