This page lists many different types of ESCs and their specifications. How do you know if yours is flash-able?
The column "FW file" names the matching .hex file for the ESC. If there is an entry, it should work (Simon has written the file for this target or someone has successfully tried it). However, if there is no entry but your ESC has an Atmega MCU it is up to you to try.
The sheet attempts to be as complete as possible but if you see an error or have additional info, feel free to contact TomSn0w for editing. As this list is based on community work, it relies on people helping to collect the data. You make it happen!
There is no guarantee if you damage your ESC or hardware. You won't be able to reset the old firmware. Errors in the sheet and the FW excepted. Also see "Notes"
ESC Spreadsheet by TomSn0w
Glossary for Spreadsheet Fields
Pads - metal contacts on the ESC's board make it easier to connect the MCU's pins to the flashing tool. For ESCs with "no" in this field, a higher level of skill or more tools are needed.
Ext Osc - whether the ESC has a dedicated external oscillator to supply a clock signal or relies on the internal clock of the MCU for timing. One or the other is needed for the ESC to measure signal pulse duration and determine the appropriate motor speed per prior calibration of the pulse width endpoints. An external oscillator may supply a more consistent timing signal and steadier motor speeds than the MCU's internal clock.