1. My hatred for Chrome (kind of recent HN influence) led me into Firefox first and foremost hence Firefox. Even though I'll be porting it to Chrom(ium-e) soon.
2. It could be my attitude but I thought writing ground up vanilla would be much easier than learning a new tool, i.e., I know Greasemonkey only as much as in its name and nothing else about its workflow.
2.1 I guess an addon is easier to distribute/install (?).
Personally, I have been using byuu's user script for this. I also love it for the click tracking removal and display of the complete url.