STR:

1. Open about:preferences#home
2. Under "Homepage and new windows", select "Custom URLs…"
3. Use Bookmark…

From the browser devtools:

Got an unexpected frameLoad in the inspector, please file a bug on bugzilla.mozilla.org! inspector.js:361:23
console.trace() inspector.js:365:23
    getMutations resource://devtools/shared/fronts/inspector.js:365
    getMutations resource://devtools/shared/fronts/inspector.js:269
    onMutations resource://devtools/shared/fronts/inspector.js:436
    onPacket resource://devtools/shared/protocol/Front.js:203
    onPacket resource://devtools/shared/client/debugger-client.js:592
    send resource://devtools/shared/transport/local-transport.js:70
    makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:111
    makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:111

We tried to reproduce this but couldn't. Given that there's a certain set of circumstances where this error is thrown, it's clear that it can happen.
We'll put it on the backlog to refer to it if we can find steps to consistently reproduce it.