I honestly can't remember, but it was quite a bit before Git or GitHub came around. I started using Linux in the late 90s, so I assume it was probably something in GNOME or some command line tool. Back then it was pretty common to just attach some diff to an email and have it committed by someone else, so even repo histories aren't always that helpful ¯\(ツ)/¯
I also remember a contribution or two to the Syllable operating system, but I can't find any reference to that on the mailing list archives anymore. In 2004 I joined the Gentoo Linux team as an "official" developer, but I started contributing in 2003.
I do know my most used FOSS contributions though:
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