Maybe Fission related. (DOMFissionEnabled=1)

Crash report: https://crash-stats.mozilla.org/report/index/144333d4-6f17-40c6-b9f5-5a9480210714

MOZ_CRASH Reason: MOZ_DIAGNOSTIC_ASSERT(cv)

Top 10 frames of crashing thread:

0 libxul.so nsGlobalWindowOuter::Print dom/base/nsGlobalWindowOuter.cpp:5333
1 libxul.so nsFrameLoader::PrintPreview dom/base/nsFrameLoader.cpp:3421
2 libxul.so mozilla::dom::FrameLoader_Binding::printPreview_promiseWrapper dom/bindings/FrameLoaderBinding.cpp:869
3 libxul.so bool mozilla::dom::binding_detail::GenericMethod<mozilla::dom::binding_detail::NormalThisPolicy, mozilla::dom::binding_detail::ConvertExceptionsToPromises> dom/bindings/BindingUtils.cpp:3299
4 libxul.so js::InternalCallOrConstruct js/src/vm/Interpreter.cpp:512
5 libxul.so Interpret js/src/vm/Interpreter.cpp:3227
6 libxul.so js::InternalCallOrConstruct js/src/vm/Interpreter.cpp:544
7 libxul.so js::Call js/src/vm/Interpreter.cpp:589
8 libxul.so js::CallSelfHostedFunction js/src/vm/SelfHosting.cpp:1558
9 libxul.so AsyncFunctionResume js/src/vm/AsyncFunction.cpp:127

Affected versions

• Firefox 91.0b2
• Firefox 92.0a1

Affected platforms

• Windows 10
• Ubuntu 20.04
• macOS 11.4

STR

1. Launch Firefox
2. Access about:support page
3. Open print preview

Additional Notes

• This crash is not occurring on 91.0b2 instead it will stay indefinitely on Preparing Preview and the Browser console will have the following message "NotSupportedError: Didn't end up with a content viewer"

bug 1714639 ?

Seems like bug 1714639

I can verify that this bug got fixed by bug 1718204 and that the regression was caused by bug 1666247.