also, there is a restart with add-ons disabled menu item, why not just a plain restart?

As silly as it sounds Mozilla has just never included a Restart button in Firefox, but the underlying code is in Firefox.

Take a look thru these extensions which add a Restart button to Firefox. https://addons.mozilla.org/en-US/firefox/search/?q=Restart&appver=&platform=windows

The Developer Toolbar (Shift+F2) has a restart command.

• "3-bar" menu button or Tools -> Web Developer

• https://developer.mozilla.org/en/Tools/GCLI

There is a restart button on the hidden/under-construction "About Profiles" page. To open that, type or paste about:profiles in the address bar and press Enter/Return to load it. The "Restart normally" button is in a gray box in the upper right corner.

Note: Be cautious in using other buttons further down this page, as their functionality may not have been completed. I suggest making a backup of your profile data before experimenting. See: Back up and restore information in Firefox profiles.

When you update the Flash plugin, Firefox usually detects the change quickly on its own without requiring a restart.

Usually if Firefox needs to update to complete an extension installation, there will be a link for that purpose above the extension on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. Then scan down the right side for one or more extensions with the relevant notice listed on them and a restart link.

If the extension was updated externally to Firefox by another program, then most likely you'll need to do a manual restart.