Tried building SeaMonkey with GCC 11.1.0, it doesn't get very far as it is not happy with the std: lib.
../dist/include/mozilla/gfx/Rect.h: In static member function 'static mozilla::gfx::RectTyped<units, F> mozilla::gfx::RectTyped<units, F>::MaxIntRect()':
../dist/include/mozilla/gfx/Rect.h:168:15: error: 'numeric_limits' is not a member of 'std'
168 | -std::numeric_limits<int32_t>::max() * 0.5,
| ^~~~~~~~~~~~~~
../dist/include/mozilla/gfx/Rect.h:168:40: error: '::max' has not been declared; did you mean 'std::max'?
168 | -std::numeric_limits<int32_t>::max() * 0.5,
| ^~~
| std::max
With lots more similar errors. Also see more demands for -Wclass-memaccess even though it is in the flags.