FoxyProxy (2.6.2) seems to leak DNS queries to my ISP, rather than route them to my SOCKSv5 provider, although network.proxy.socks_remote_dns is set to 'true', and even if no other extension is installed.
On the other hand, if FoxyProxy is disabled (but SOCKS is set in the Tools|Options|Advanced|Network setting, DNS queries do not leak.
How can I use FoxyProxy without leaking DNS queried to my ISP?
You need to check the "Use SOCKS proxy for DNS lookups" option in Global Settings->Miscellaneous. network.proxy.socks_remote_dns has nothing to do with FoxyProxy.
I'm sorry - whatever the relevance of socks_remote_dns is (nothing, as you pointed out), "Use SOCKS proxy for DNS lookups" *is* checked, and still, FoxyProxy seems to be leaking DNS queries: simply disabling FoxyProxy stopes any leak.