I'm reading this on ChromeOS too, but on Chrome. I've tried using Firefox a couple years ago on this machine, but it was the mobile variant from the Android play store which felt a lot like running a mobile app on a large laptop screen, which made everything feel a bit awkward. Maybe it has gotten better since then?
The Play Store version is definitely just running the mobile app version on a large laptop screen. This version is just using the ability to run Linux apps to run the normal desktop version of Firefox directly (albeit in a VM like all the Linux/Android apps on ChromeOS). I'm not sure if hardware acceleration is enabled for the Linux VM for video/graphics things yet though so I'm not sure about how well it performs.
It does. CTRL+T, CTRL+Shift+T, CTRL+L all work. FF mobile doesn't have Dev tools, but Kiwi, a Chromium fork browser, does, so CTRL+Shift+I will bring it up.
My Pixelbook has been collecting dust ever since I switched to my Framework, but back when I was using it I recall Firefox working damn near perfecfly via Crostini. Only quirk was that ChromeOS obviously expects a different browser, so getting other software to reliably open links in Firefox instead of the built-in Chrome was less than foolproof, but it otherwise worked exactly as well as on an ordinary (GNU/)Linux desktop system.
I ran Firefox in Crostini once. I'd say it was an "eh" experince. But then I installed Linux on it. Too bad that the way I installed Linux on it is being discontinued...
I'm reading this on ChromeOS too, but on Chrome. I've tried using Firefox a couple years ago on this machine, but it was the mobile variant from the Android play store which felt a lot like running a mobile app on a large laptop screen, which made everything feel a bit awkward. Maybe it has gotten better since then?