Debugging Fennec can be trickier than Firefox because it uses multiple processes.
For example to debug reftests in fennec on desktop linux the following recipe works:
python runreftest.py --debugger=gdb TEST_PATH/reftest.list
once inside gdb
# break on fork calls (gdb) catch fork # break on exec calls (gdb) catch exec (gdb) run #the first fork will be for glx probing and can be ignor (gdb) continue #the next fork is the content process, make sure we follow into it (gdb) set follow-fork-mode child (gdb) continue #this will break on the exec (gdb) break some_function_in_the content process