The Android browser used to implement it, before Chrome for mobile came along. I think text reflow appeals to a certain kind of power user but is confusing for most users. So it sounds like a good feature for Opera, but not for mainstream browsers.
Here's the bug report from firefox about this issue[1].They removed it because it's slow and because they feel that today page authors use large enough text.
So maybe the sensible thing to do is to convince the firefox team to enable it as an option.
HTC Sense 6 browser still implements it pretty well. I still use it despite it being somewhat slow and annoying in other ways, because at least I can read with it.
