flashfxp.5.1.x-patch.exe - for versions 5.x builds 3805 - 3825, will work on the next versions, is unknown. To register, use the key from the patch.
Alternatively, you can use this key without the patch, with blocking network:

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Note:  To Be On Safe side
1-install & register offline 
2-auto updates must be disabled
3-add these entries to your hosts

127.0.0.1 secure.flashfxp.com 
127.0.0.1 promo.flashfxp.com 
127.0.0.1 liveupdate.flashfxp.com
127.0.0.1 flashfxp.com

4-Create Outbound Block rule to for Following IP in FW

96.30.5.209

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flashfxp.5.1.0.3820+patch_n_5.x_trialreset - toolkit for reset trial.
Buttons:
'Export dll' - save msimg32.dll, which automatically resets the trial (for use, puts/export it
to folder with FlashFXP.exe, each time you start the program will be reset trial).
'Reset trial' - for manual reset trial if you do not use msimg32.dll.
'Patch nag' - patching FlashFXP.exe, to remove the starting nag (as well as in the patch above, the work on next
versions can not be guaranteed, unlike flashfxp.5.1.x-patch.exe, only works for versions 5.1.0.3820 and later).

Advanced - loaders, if someone wants to do without the patch FlashFXP.exe.
loader - little updated one that previously laid out - added (on the off-chance) removal cert.dat.
loader_for_dll - loader without resetting the trial (only removes nag), for use in paired with the above described msimg32.dll.