RR is the new project and isn't a web-debugger but was developed in the process of debugging Firefox issues. RR is short for record and replay, two strengths of this debugger compared to other debuggers. RR is aimed at C/C++ debugging of Firefox on Linux at the moment. While RR is being improved upon, it's not yet recommended for general purpose C/C++ debugging outside of Firefox on large projects. RR lets the developer use GDB to debug during the replay process, of a record captured by RR of a series of program sequences.
The RR project site explains, "rr aspires to be your primary debugging tool, replacing — well, enhancing — gdb. You record a failure once, then debug the recording, deterministically, as many times as you want. Every time the same execution is replayed."
Other details on RR can be found via this blog post by Robert O'Callahan.