Yes, I noticed this too. At this point in life I've partially given up on having a proper desktop experience in MicroB as it is now, and just hope that one day, some brave and able soul will figure out a way to make a newer version of Gecko work with MicroB (either the current closed-source-UI thereof, or by recoding the closed-source parts to get it to work).

For the record, it's happening even though I have my MicroB report a completely stock Firefox useragent. So maybe they're just redirecting all oldish browsers to the mobile page...