Riot app unusably slow with Firefox (or Waterfox) in safe mode

[grahamperrin]

Description

Whilst attempting to troubleshoot a different issue, I attempted to use Riot with Firefox in safe mode. I discovered that the app is unusable with the browser in that mode.

Steps to reproduce

• start Firefox or Waterfox in safe mode
• try to load Riot web e.g. https://riot.im/app/

On a relatively old, slow machine (Ergo Vista 621, 4 GB memory) with Firefox 57.0.1 (64-bit) on Kubuntu:

• Riot never becomes usable.

On a relatively modern, fast machine (HP EliteBook 8570p with 16 GB memory) with Waterfox 56.0.1 and Firefox 56.0.2 (64-bit) on FreeBSD-CURRENT:

• after an extraordinarily long wait, Riot might load but periods of usability (interspersed with long periods of the app not responding) seem to last no longer than a second or two

– and if multiprocess is disabled (e.g. browser.tabs.remote.force-disable) then the entire browser is non-responsive for terribly long periods. Yesterday, for example:

Firefox 56.0.2 … just responded, momentarily, after around twenty-two minutes.

Logs can not be sent, because the application is unusable.

[lampholder]

Clicking the Start in Safe Mode button will temporarily disable your extensions and themes, turn off hardware acceleration and reset toolbar and button customizations. When you leave Safe Mode and start Firefox up normally, your extensions, themes, and settings will return to the state they were in before you entered Safe Mode.

I would guess that safe mode might hide state in localstorage/indexeddb forcing a full resync. Coupled with the lack of hardware accelleration maybe, plus general worse performance on FF (still), this might be causing this problem.