1: proxy manually configured in Firefox preferences, no PAC file.
2: Use the PAC file found at the URL given by the value of the "network.proxy.autoconfig_url" preference.
4: Use WPAD to get the PAC file.
5: Do whatever the OS says to do in terms of proxy settings (including what PAC file to use, if any).
The default value is 5. I believe setting the preference to 0 will ensure that you always use a direct, not proxied, connection to the target site, and in particular that no PAC file is used.
Edit: Ah, just noticed that kuschku points to UI for all this stuff below too.
Yes. In Firefox, the "network.proxy.type" preference is an integer that can take on the values 0, 1, 2, 3, 4, 5 with the following meanings:
0,3: no proxy (see https://bugzilla.mozilla.org/show_bug.cgi?id=115720 for the history on why 3 is the same as 0)
1: proxy manually configured in Firefox preferences, no PAC file.
2: Use the PAC file found at the URL given by the value of the "network.proxy.autoconfig_url" preference.
4: Use WPAD to get the PAC file.
5: Do whatever the OS says to do in terms of proxy settings (including what PAC file to use, if any).
The default value is 5. I believe setting the preference to 0 will ensure that you always use a direct, not proxied, connection to the target site, and in particular that no PAC file is used.
Edit: Ah, just noticed that kuschku points to UI for all this stuff below too.