STR:

• open http://techno-barje.fr/fission/duplicated-url-scripts/
• navigate the same tab to google.com
• open the Browser Toolbox
• navigate back to the first page

You will see the tab crash, with the following exception:
https://crash-stats.mozilla.org/report/index/b900cae9-334c-4b1e-a28d-183720211214

Thread 30 "DOM Worker" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7f91b3aff700 (LWP 128175)]
0x00007f91bbdbc225 in js::ContextChecks::check(JS::Value const&, int) () from /mnt/desktop/gecko-dev/obj-firefox-artifact/dist/bin/libxul.so
(gdb) bt
#0  0x00007f91bbdbc225 in js::ContextChecks::check(JS::Value const&, int) () at /mnt/desktop/gecko-dev/obj-firefox-artifact/dist/bin/libxul.so
#1  0x00007f91bbe0496c in JS_CallFunctionValue(JSContext*, JS::Handle<JSObject*>, JS::Handle<JS::Value>, JS::HandleValueArray const&, JS::MutableHandle<JS::Value>) ()
    at /mnt/desktop/gecko-dev/obj-firefox-artifact/dist/bin/libxul.so
#2  0x00007f91b97e0749 in mozilla::dom::(anonymous namespace)::DebuggerImmediateRunnable::WorkerRun(JSContext*, mozilla::dom::WorkerPrivate*) ()
    at /mnt/desktop/gecko-dev/obj-firefox-artifact/dist/bin/libxul.so
#3  0x00007f91bcbe7abb in mozilla::dom::WorkerRunnable::Run() () at /mnt/desktop/gecko-dev/obj-firefox-artifact/dist/bin/libxul.so
#4  0x00007f91bcbe3ca7 in mozilla::dom::WorkerPrivate::DoRunLoop(JSContext*) () at /mnt/desktop/gecko-dev/obj-firefox-artifact/dist/bin/libxul.so
#5  0x00007f91bcbda3e9 in mozilla::dom::workerinternals::(anonymous namespace)::WorkerThreadPrimaryRunnable::Run() ()

This crash relates to this Javascript code of DevTools:
https://searchfox.org/mozilla-central/rev/71befbc9bc348d33f0c7307dc48e3cb36ba78650/devtools/shared/DevToolsUtils.js#48

exports.executeSoon = function(fn) {
  if (isWorker) {
    setImmediate(fn);

When being called from the thread actor, on new worker sources:
https://searchfox.org/mozilla-central/rev/71befbc9bc348d33f0c7307dc48e3cb36ba78650/devtools/server/actors/thread.js#2012-2023

I suspect that the worker is being destroyed in middle of this setImmediate call.

Just tesed and I can easily reproduce the crash.