You should be able to create a file in the TEMP path (many apps, like UR itself, need to be able to write there, so you should be able to as well). Create a text file there named UltraRecall.loc with contents of the full path+filename of UltraRecall.exe (with no quotes or newlines). Literally type %TEMP% into Explorer or a file dialog to save in that path.
Or force UR to run in portable mode (see
http://www.kinook.com/UltraRecall/Ma...stallation.htm) and start UR before using the Firefox extension.