Browser rendering engine skills directly translate to native front-end development skills, especially on mobile. Not to mention that writing an engine makes you an expert in the performance model pertaining to Web development—instead of having to guess how CSS properties perform like most people do, you just know.

Does being a MySQL hacker make you unqualified for Web app backend code or database administration? I don't think so.

I'm guessing they are skilled and versatile C++ programmers with years of experience even on difficult and restricted platforms.

Where could they not go?

Oh come on. It's not like these guys went to Browser Rendering U, and that's all they know!

No, but several of them have been working on rendering engines since before Mozilla was founded.

Several of them? Such as?

Why do you think they'll want to work for Mozilla "on the cheap"?

If a competitor fires people with the skills you need, the hiring process gets a lot easier and as such cheaper.

It not necessarily means low wages, but more "easy grab".

"easy grab", exactly. Hiring ex Opera employees who worked on a rendering engine will be cheaper than hiring c++ coders who then have to ramp up knowledge on how to code a rendering engine.