Sometimes freezes Firefox when in about:addons

I am testing it out with Firefox Developer Editition for debugging my add-on.

So when I am in `about:add-ons` it often freezes Firefox. Also, it ignores my breajkpoint.

STR:
1. Debug add-on.
2. Set breakpoint somewhere in code that is to be called in the settings.
3. Go to settings page of add-on.
-> First issue: Breakpoint does not trigger, but is just ignored.
4. Press F5 in the browser to reload the options page OR click on "extensions" on the left to get to the overview.
-> Freezes.

Firefox log:
```
JavaScript warning: resource://devtools/shared/base-loader.js -> resource://devtools/server/actors/inspector/walker.js, line 1612: DebuggeeWouldRun: debuggee `self-hosted:582' would run
JavaScript warning: resource://devtools/shared/base-loader.js -> resource://devtools/server/actors/inspector/walker.js, line 1612: DebuggeeWouldRun: debuggee `self-hosted:582' would run
JavaScript warning: resource://devtools/shared/base-loader.js -> resource://devtools/server/actors/utils/event-loop.js, line 118: DebuggeeWouldRun: debuggee `chrome://global/content/bindings/general.xml:15' would run
```

Firefox 62

Start by reproducing the documented steps in Firefox Developer Edition 62: debug an add-on, set a breakpoint in the settings code, open its settings page, then reload or return to the extensions overview. Use the provided Firefox warnings to investigate why the breakpoint is ignored and why the browser freezes; done means the breakpoint triggers and these navigation actions no longer freeze Firefox.

Written by the indexing model from the issue text.