Unlike for the default download location there is no place in the normal preferences to change the default upload location; you will need to dig a little deeper:

To open the internal settings list, click on the location bar and enter about:config (just like that, no http:// or the like), hit the enter key. If prompted "to be careful", do so and proceed. Search for the setting dom.input.fallbackUploadDir (the list is sorted alphabetically), click on its little pen to the right to show the text box that lets you enter the path to the desired default directory.

Note that Firefox will store the last used upload location for every site; fallbackUploadDir works only for sites where you have not uploaded anything so far.

Hi Sinsang, thank you for your quick solution to the problem. I use a local java application with the FF and keep opening different import files. it was annoying to me to search the folder every time after starting the application. With your help this is no longer necessary. Many greetings

hessline