Well that's unfortunate to hear :( Hoping Arch can fix releasing debug symbols for official binary builds soon... but I don't expect it to happen soon.

Also how does unwinding info help during debugging? Does gdb use it? Do I need special reverse-engineering tools to extract useful information?

Arch already uploads symbols for their Firefox builds so that's OK, it's the dependencies that are missing. Unwinding information is used to produce accurate stacks. If it's lacking our unwinder has to rely on heuristics to discover the frames on the stack and those might end up producing suboptimal traces.