From Forum Nokia Wiki

I think you mean "...when built with S60 3rd Edition SDK, supporting Feature Pack 1."

However, this SDK should NOT be used to build apps for S60 3rd Edition devices to start with and this his only the visible tip of the iceberg, there are other issues that cannot be simply solved by adding a flag to a tool call.

S60_3rd_FP1 not compatible with S60 3.0 devices?

You said S60_3rd_FP1 should NOT be used to build apps for S60 3rd Edition devices.

Is it better to stay using S60_3rd_MR when building applications for devices like Nokia N73 or Nokia E50 ?

It would be interesting to see, where this is officially stated, as I have totally missed this point. I have thought S60_3rd_FP1 is binary backwards compatible with S60_3rd_MR, when not using the APIs especially marked as incompatible. (s60_3rd_fp1_sdk_f_ReleaseNotes.txt, 2006-10-11)

However, based on your comment, I am now considering to stay with S60_3rd_MR and interrupt my current project of adapting to S60_3rd_FP1.

Use S60_3rd_MR for S60 3.0 devices

Answering to myself:

I was pointed to this article: http://wiki.forum.nokia.com/index.php/How_do_I_start_programming_for_Symbian_OS%3F

There chapter 2 quite clearly says, that you should use S60 3rd SDK MR for "Developer Platform: S60 3rd Edition" based devices and S60 3rd SDK FP1 only for "Developer Platform: S60 3rd Edition, Feature Pack 1" based devices and newer.

So I shall try to stay with S60_3rd_MR to be sure that my applications safely run in the widest range of S60 3rd Ed devices.