I have several tab groups (~ 20) in Firefox. I can start the browser without any problems. However, as soon as I …

• click at the "Group tabs" icon in the toolbar, or
• right-click on a tab and hover over "Move to tab group",

… Firefox becomes unresponsible/freezes for a rather long time (more than 2 minutes). It seems to load all tab groups (it doesn't load all the pages! I deactivated this in the settings). While this is happening, I get several "Unresponsive script" warnings, like:

• Script: chrome://global/content/bindings/tabbox.xml:0 (most of the time)
• Script: chrome://global/content/bindings/tabbox.xml:418
• Script: chrome://browser/content/tabview.js:400
• Script: chrome://browser/content/tabview.js:522
• Script: resource://modules/sessionstore/SessionStore.jsm:3578
• Script: resource:///components/PageThumbsProtocol.js:79 (rare)
• Script: resource://gre/modules/XPCOMUtils.jsm:323 (rare)
• (probably also other warnings, didn't record them yet, though)

On all of these I click "Continue". After ~ 2-3 minutes and 3-5 warnings, I can use Firefox again. Now I can switch tab groups without any problems.

Why is this happening? How can I prevent the long loading time? Is there maybe a about:config setting I could try?

I started Firefox in Safe Mode (= without any add-ons): the problem still exists.