We had to choose between Webkit and Gecko. For us it was primarily the license, which was GPL vs MIT. Gecko was a non starter.
That's not what "viral" means. MPLv2 cannot "infect" your own code like the GPL can just by virtue of using the library.
The only condition is if you want to make modifications to MPLv2 licensed code files, you do have to retain the license on all contributions.