I've been looking into the firmware update. There is not a newer version of the Tea firmware than what came pre-installed (T740.5.17.1680) although there are newer versions available from Pocketbook. Pocketbook doesn't support putting the Pocketbook firmware on Tea-branded hardware, but according to this thread (for a different Pocketbook model)
https://www.mobileread.com/forums/sh...d.php?t=231431
the difference is that the Tea firmware points you at the bookstore you bought the reader from, e.g. decitre.fr in my case. Since I'm not likely to ever use that, I might try updating to the Pocketbook firmware anyway at some point if there is no official Tea firmware update meanwhile, but I can't say that I've run into any horrible bugs yet that make me want to rush to do that. OTOH, all I've been doing with the device so far is side-loading EPUBs from Calibre and reading them.
BTW, one odd thing I found by trial and error: it appears that it only goes into USB drive mode when you have a book open. If I plug it in when it's at the home screen, it goes into battery-charging mode instead even if I've selected the setup option to always go into USB drive mode.