hi, incompatible addons that you install can disable the multiprocess mode - the https://addons.mozilla.org/firefox/addon/add-on-compatibility-reporter/ makes them visible in the addons manager.

I run firefox in safe mode, instead of installing another addon, and it still launched with one instance. Besides, as I said, nothing has changed, I didn't install any new addons, and non of the current ones updated, so everything is the same as yesterday.

what does about:support say about the state of multiprocess?

I found two mentions of it (I have non english version of firefox). The first line from the picture says something like this: Multiprocess windows: 0/2 (disabled by availability tools)

And the other line says "true" in the "Enabled" column.

ok, please see https://wiki.mozilla.org/Electrolysis/Accessibility

Ok, not much I understand from this. BUT:

Tools that can trigger disabling of electrolysis include:

• Vision impaired related assistive clients (screen readers, magnifying glass clients)

Yesterday, by mistake, I launched the magnifying glass tool in Windows (I pressed Win key + NUMPAD+ by mistake), and closed it. So now you are telling me this disabled multiprocess in firefox for me? What can I do to enable it? Can I clear these two?

• accessibility.lastLoadDate
• accessibility.loadedInLastSession

yes, right-click and reset those two preferences and create a new integer preference called "accessibility.force_disabled" and set its value to "1".

Ok, it worked. Thank you very much.