Copy the URL of the proxy configuration file (.PAC file) and go to the URL with Firefox.
Firefox will either:
- immediately open the file in the browser or
- ask with which program to open it. Open the file with a text editor of your liking.
In the PAC file, search for "PROXY". Next to it, you should find the address of the proxy server used in your company. You can use that information
proxyserver:port in the Dropbox proxy settings.
If you have several proxy servers defined in the PAC file, check them all until you find the right one that let Dropbox access the Internet.