Firefox

Used by: Firefox, Thunderbird, Tor Browser.

Helper library

The checker runs this engine's own PAC helper library, copied verbatim at 3f73c528a1ae (MPL-2.0). Upstream source.

Version ranges and quirks

Later ranges list only what changes compared with the range before.

Firefox (all)

Profile of current Firefox releases; older versions are not modelled separately.

Behaviour	Value	Source
alert() output	`console`	code, verified 2026-10-04
Non-ASCII result rejected	`false` The returned string is used as is; non-ASCII characters are not rejected.	code, verified 2026-10-04
DNS lookups	`sync`	code, verified 2026-10-04
dnsResolve is IPv4 only	`false` Whether dnsResolve can return IPv6 addresses in current Firefox has not been checked.	expert, unverified
JavaScript level accepted	`latest`	code, verified 2026-10-04
Calls FindProxyForURLEx if defined	`false`	code, verified 2026-10-04
Microsoft *Ex functions	`false`	code, verified 2026-10-04
Global variables persist between calls	`true`	expert, unverified
Host lower-cased by the engine	`true` The host comes from the parsed request URL, which is lower-case.	expert, unverified
myIpAddress() source	`routing_ipv4`	code, verified 2026-10-04
Native (C++) bindings	`dnsResolve, myIpAddress, alert`	code, verified 2026-10-04
On exception	`direct` If FindProxyForURL throws, the request goes DIRECT.	code, verified 2026-10-04
On non-string / undefined result	`direct` A result that is not a string (for example a missing return) makes the request go DIRECT.	code, verified 2026-10-04
URL passed to FindProxyForURL	`all_path_query` Firefox removes path and query for every scheme, not only https (preference network.proxy.autoconfig_url.include_path, default false).	code, verified 2026-10-04

Function availability

Derived from each engine's vendored helper library plus its native bindings (see engines).
Function	chromium <=51	chromium >=52	firefox all	pacparser <=1.4.6	pacparser 1.5.0	pacparser >=1.5.1	reference all
`isPlainHostName`	yes	yes	yes	yes	yes	yes	yes
`dnsDomainIs`	yes	yes	yes	yes	yes	yes	yes
`localHostOrDomainIs`	yes	yes	yes	yes	yes	yes	yes
`isResolvable`	yes	yes	yes	yes	yes	yes	yes
`isInNet`	yes	yes	yes	yes	yes	yes	yes
`dnsResolve`	yes	yes	yes	yes	yes	yes	yes
`myIpAddress`	yes	yes	yes	yes	yes	yes	yes
`dnsDomainLevels`	yes	yes	yes	yes	yes	yes	yes
`shExpMatch`	yes	yes	yes	yes	yes	yes	yes
`weekdayRange`	yes	yes	yes	yes	yes	yes	yes
`dateRange`	yes	yes	yes	yes	yes	yes	yes
`timeRange`	yes	yes	yes	yes	yes	yes	yes
`alert`	yes	yes	yes	no	no	yes	yes
`dnsResolveEx`	yes	yes	no	yes	yes	yes	no
`myIpAddressEx`	yes	yes	no	yes	yes	yes	no
`isResolvableEx`	yes	yes	no	yes	yes	yes	no
`isInNetEx`	yes	yes	no	yes	yes	yes	no
`sortIpAddressList`	yes	yes	no	no	no	no	no

Rules with notes for firefox

PAC-C018Upper-case letters in a hostname pattern
host is passed lower-cased (URL canonicalisation). doc, unverified
PAC-C027Cloudflare Gateway proxy endpoint with PROXY instead of HTTPS
Supports the HTTPS proxy type through a PAC file (Cloudflare lists Firefox as supported). doc, verified 2026-10-04 · ref
PAC-D003No Cache-Control header on the PAC response
Reported to keep the PAC for the session when no caching headers are present. expert, unverified
PAC-K001ES2015+ syntax (let, const, arrow functions, template literals)
SpiderMonkey; current ECMAScript syntax parses. expert, unverified
PAC-K002alert() used in the PAC
Message is written to the browser console; no dialog. expert, unverified
PAC-K003IPv6 extension functions (isInNetEx, dnsResolveEx, myIpAddressEx)
Ex functions are not implemented. expert, unverified
PAC-K005SOCKS keywords mean different versions on different engines
Reported to treat plain SOCKS as SOCKS5; versioned keywords unverified. expert, unverified
PAC-K006HTTPS proxy keyword without a PROXY fallback
HTTPS keyword supported in current versions. expert, unverified
PAC-K013Routing decided by myIpAddress()
Reported to resolve the local hostname (gethostbyname(gethostname())). expert, unverified
PAC-K015Regular-expression syntax inside a shExpMatch pattern
Same helper-library approach (ascii_pac_utils.js); regex alternation works. code, unverified
PAC-P001DNS lookup on every request
dnsResolve is synchronous on the PAC thread. expert, unverified
PAC-X001No unconditional return at the end of FindProxyForURL
undefined is treated as DIRECT. expert, unverified
PAC-X013PROXY entry without a port
Reported to reject the entry; needs lab confirmation. expert, unverified