Just one kind of firmware for the M3?

Am I right that there is just one kind of firmware for the M3? Compared to the M6 where we have S... and T... depending on the display...

Yeah. It doesn't matter they are all compatible with all M3's. But be careful becasue sometimes upgrading formats the player depending on what you are upgrading from and to. For example upgrading from 2.003.2 to 2.003.5 or from 2.004.3 to 2.004.5 does not format the player but going from 2.003.5 to 2.004.3 does.

Aha...
So is this trial and error to find out?
And where could I get older firmwares...?

Search meizume for older firmware. I just checked the sticky for M3 2.004.3; as of this post the firmware's still there for download.

It's not trial and error to find out whether your M3 will be formatted by firmware update. There's been lots of discussion about this. Essentially the M3 will re-format when updating between 2.00x releases (e.g., 2.003.x --> 2.004.x). It won't re-format when updating between 2.00x.x releases (e.g., 2.004.3 --> 2.004.5).

In any case you'd be prudent to save a backup from the M3 to your computer.

Harry

Yes, I agree with hjacobson. Most upgrades (as explained above) will not format your player, but it is better to have your files backed up anyway. I have never had any problems with it formatting when it is not supposed to, but that is not to say it doesn't happen.