chore: added ff cheat sheet

Author: pavelfeldman

browser_patches/firefox/cheatsheet.md

@@ -0,0 +1,15 @@
+### Debugging
+
+#### Stack trace
+
+It `mozglue/misc/StackWalk.cpp` add 
+
+```c++
+#define MOZ_DEMANGLE_SYMBOLS 1
+```
+
+In native code use
+
+```c++
+nsTraceRefcnt::WalkTheStack(stderr);
+```