Saving downloaded files to specific folders on my HD

I would like to be able to save files that I download with Firefox to folders that I have created on my hard drive. For example: When I click on a link in Firefox to download a .pdf, I would like to specify the exact location that the file gets downloaded to ie the following path /Macintosh HD/users/dallas/documents/pharmacology , the destination folder "pharmacology" is one that I created previously. Right now what happens when I click on a link to download a file is a window comes up asking me where I want to save the file, but the drop-down menu where I select the location for the file to be saved only allows me to save the file to the "documents" folder. It does not allow me to specify any of the subfolders within my "documents" folder. Is there any way to save files to the specific subfolders that I previously created? Any help is appreciated.