You may find this hard to believe, but I recommend that you read this post, “Spread Firefox but don't be a fanatic,” from the Firefox marketing community web site:
It begins with this:
In my opinion it's ridiculous to have “loyalty” to a company. I base my evaluation of products only on their own self-worth and merit as a product, and not on any pre-conceived opinions about the company that develops it or distributes it. Generalizing about the software of one company as a whole is, frankly, narrow-minded and readily worthy of ridicule. A comment such as, “Note also that Firefox users are in denial about the greatness of Microsoft's products,” would display a bias for Microsoft products and discredit whoever said it. Both sides are doing this, and often our side is a bit ruder about it. Of course you can often expect that if one application from a company is good that they have applied the same level of standard to their other applications, but you still have to try them out to see what you think of them.
There are many valuable thoughts in this entry about fanaticism (vis-a-vis evangelism).