That's actually useful information. There are 750FTE at Mozilla. How many of those do you think would it take to do the best job possible on the browser. That would give one way of calculating a budget.
Hard to say, and I suck really bad at estimates. If I low-ball it at maybe 200-300, that's likely off by a factor or three. A possibly old but close enough page on chromium.org lists the following teams:
Animations Team
Binding Team
Device Team
DevTools
DOM Team
Ecosystem infra
Input Team
Layout Team
Paint Team
Rendering Core
Speed Metrics Team
Style Team
Web Capabilities (Project Fugu)
Worker Team
and that looks like it's only focused on Blink, not including the actual chrome (desktop/mobile apps) or V8 which would both be substantial efforts as well. Who knows how much technology is pulled from other teams at Google, like video encoding for WebRTC, server performance collaboration for the SPDY/QUIC effort, low-level storage for IndexedDB, devrel with buggy websites or certificate authorities, security audits and whatnot?
For doing the "best job possible", you can probably blow up team size by a lot, there are numerous tasks that would benefit from more attention and I'm sure Mozilla are already making hard trade-offs at their current staffing levels (which obviously include a lot of people outside of Firefox engineering).
Interesting that 'factor of three' puts it within the range of the number of employees Mozilla currently has. That means that the browser is already in danger if they do other stuff besides. Hm. That's more urgent than I thought it would be.
To clarify, I wrote "factor OR three", by which I meant to emphasize my complete lack of certainty regarding the "best possible" performance you asked about. I'll only commit to my earlier statement about the definite irrelevance below 100 people.
