From MozillaZine Knowledge Base
When using a proxy auto-config (PAC), if the primary proxy becomes unresponsive, Mozilla applications will use the secondary proxy and not retry the primary for a specific period of time. This preferences specifies that period of time.
Possible values and their effects
The number of seconds to wait before attempting to recontact the unresponsive proxy server. (Default: 1800, or 30 minutes)
First checked in
Has an effect in