I get a big jank when I:

1. Running the browser for a while, maybe using other apps though.
2. Focus a browser window
3. Ctrl-T for new tab.
4. Begin typing in the address bar, I get the first letter typed.

Jank happens, 3-5 seconds.

5. can type the rest of what I wanted and move on with my life.

I am using Firefox Nightly on LInux Mint, I have fission enabled.

I have a profile: https://share.firefox.dev/2F9X3Lu

It points to some localisation stuff, the jank involves a lot of Localisation code including calls into Fluent. Before the jank begins there's _rebuildEngineList which AIUI is about the search engines that are involved in the address bar, then FlushPendingTranslations is called on a notification which appears to create a promise that causes the rest of the localisation jank to run.

I don't know if running that localisation work is a good idea, or should be done at a different time. But it shouldn't cause user input to jank. Or maybe it's supposed to be shorter-running so it's only a tiny bit of work.

This looks like a ::Search problem, where the _rebuildEngineList code lives. In case there's nothing that can't be done there, I guess it can be moved back.

This profile shows the same issue as bug 1642415.

Paul, can you attach about:support data from the profile where you're seeing this?

Here is about:support, thanks.

(In reply to Paul Bone [:pbone] from comment #3)

Created attachment 9169800 [details]
aboutsupport.json

Here is about:support, thanks.

Uninstall the old (Firefox 77) en-US langpack your profile somehow has, and the problem will go away.

Thanks, I can do that, but how do everyday users who are experiencing this find this information/steps. Maybe if this is always a good idea the browser should do it for them?

Also that was the only language, now there are no languages. An everyday user might assume their browser doesn't know how to make any text now. (i realise that these are localisation poblems, not search problems). I'll NI flod.

Thanks Gijs.

This is an issue that happens only for a very small set of users that installed a language pack for 77, with incorrect compatibility, and AFAIK there's no easy way to fix it (ignoring the size of population). All details are in bug 1646016 and siblings.

(In reply to Paul Bone [:pbone] from comment #6)

Also that was the only language, now there are no languages. An everyday user might assume their browser doesn't know how to make any text now.

There is always a language, that's the one of the build. Most users (94%, according to telemetry) will never install another language.