At that time, I had a fair amount of free time (this was around the time my summer internship ended but before school started). I downloaded the source code to Thunderbird and then built it, only to discover that every time I built it, it crashed in a linker. After bugging people about it on IRC, I learned that I needed a swap file, and I finally managed to get a successful build. Then, having had programming experience, I decided just to fix the bug myself. I honestly can't remember for certain, but I believe I built the original patch without much aide from people on IRC--it was only the inability to build that I had needed to ask for help.
I do remember that I needed guidance on how to get a patch committed. This being mailnews code around the time that Mozilla Messaging was being set up, the pool of potential reviewers and superreviewers was rather slim. I had been advised that David Bienvenu and Scott McGregor were my best candidates for review. Unfortunately, around this time, Scott had cut off all work with Mozilla; after two months of not getting any response from him whatsoever, I switched to another reviewer.
While I was waiting for the patch to be reviewed, I recall Dan Mosedale talking about bug 132340 indirectly; later, when I asked if there was a fix that people would be interested in, that bug was what I was pointed towards. This patch took about two months and four review cycles to be accepted, but it eventually satisfied my reviewer and superreviewer and became my first contribution to Mozilla. That first patch I worked on? It turned out to be significantly more complicated than I first appreciated and finally was closed 9 months after I started work on it. After bug 132340, I started getting more and more involved with Mozilla development and have been a contributor since then.