I got the same problems (setup the pref's every startup; could'nt download to the desktop or another folder, which is'nt the sharing folder; furthermore i could'nt use the german user surface).
I read about this problem in other forums too.
May i got a workaround, terribly i can't exactly remember, how i did it (means the steps order).
The 2.6.5 version didn't known this problems, so i got the idea to use these preferences.
In every case it has to do with the
"~/library/preferences/.limewire/limewire.props"
document( invisible), which should be used from the 2.6.5 (possibly previous versions too ).
I think it was this way:
1. I uninstalled the whole old limewire by using the "Uninstall LimeWire" application.
2. I installed the 2.6.5 , started it one time including doing the pref's setup, then I quit it.
3. I installed the 2.7.3, started it and it used the 2.6.5 pref's without any problem.
4. I removed the whole 2.6.5 folder and doc's except the above mentioned, the
"~/library/preferences/LimeWire_Installer.plist"
and the
"~/library/preferences/LimeWire.plist"
("~" means your home directory).
Do this step by drag and drop !!!.
Afterwards the 2.7.3 worked and started without problems till now.
Maybe I exchanged step 3 and 4, i don't know anymore, so I please you to try different orders of the steps above to get the ultimate solution.
Unfortunately I got problems with my mail-account this time, so it would be nice, if anybody posts in this forum, if it works.

Hope, it helps you, folks.
( sorry for my terrible english)