Freeformat is a rare mp3 option that allows higher and non-standard bitrates.
Only a few decoders support such files.
-ff -ignto decode any freeformat bitstream.
Probably not. Freeformat is an option supported by the Lame encoder which allows the use of non-standard bitrates, up to 640kbps. However, the mp3 spec states that decoders aren't required to support bitrates above 320kbps when in freeformat mode (Thanks Gabriel Bouvigne for that information).
A series of mp3 files were generated using the lame encoder. Free format bitrates of 300kbps, 400kbps, 500kbps, and 600kbps were used. Most decoders rejected these mp3s (black in the table above). Those that did produce sensible output are shown in blue in the table above. There were no "half correct" results - all successfully decoded files sounded identical to the original .wav, and an aural assessment was all that was performed.
I only tested multiples of 100kbps from 300-600. A free format bitstream can be at any integer number of kbps below 640kbps, so there are hundreds of further possibilities. The lame documentation states "Decoders which can handle free format: MAD supports up to 640kbps; lame --decode supports up to 550 kbps; Freeamp supports up to 440 kbps; l3dec supports up to 310 kbps." At 600kbps, lame decoded the right hand channel correctly, but the left channel was noise. Lame is changing continuously, so you may get different results in the future.