> Is there some flag to nuke that feature from eg Firefox?

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.