In bug 1558840 we exposed the DNS suffix list on Windows. However, this only extracts the connection specific DNS suffix [1], not the search list you can also configure.
Initially I thought you can get the list via FirstDnsSuffix which is a linked list [2], but it seems that's not true. Instead, it seems it's only available from the windows registry.

[1] https://simpledns.com/kb/63/how-to-configure-dns-suffixes-on-windows-vista-2008
[2] https://docs.microsoft.com/en-us/windows/win32/api/iptypes/ns-iptypes-ip_adapter_addresses_lh

https://hg.mozilla.org/mozilla-central/rev/a55bd3800cdf

This issue is verified fixed using Firefox 72.0a1 (BuildId:20191121214422) on Windows 10 64bit and Windows 7 64bit. Firefox parses the DNS suffix list from SYSTEM\CurrentControlSet\Services\Tcpip\Parameters successfully.

Valentin, is this a good candidate for a beta uplift? Since the code for Bug 1558840 is available in Beta 71?

(In reply to Emil Ghitta, QA [:emilghitta] from comment #4)

Valentin, is this a good candidate for a beta uplift? Since the code for Bug 1558840 is available in Beta 71?

Likely not, since the registry suffix list is less common than the per-network suffix. Also because of bug 1597137 comment 5 which might require backing out bug 1582472.