This isn't just for some power users- it will increase their share among regular people whose pages will load even faster making Firefox popular.
Or are they waiting until the user share falls below 5%? Maybe they should listen to Andy grove and prepare now.
It might mean millions of FF users would suddenly struggle with captchas, but it might also mean that site creators just stop using reCaptcha and similar.
The engineering effort required to implement this feature is better spent in doing useful things.
Despite all Microsoft's posing and snuggling up to the Open Source world, they're just as bad as Google and Facebook if given the chance. (And they have been as bad in the past.)