When tab groups are loaded, Firefox becomes unresponsible for minutes (Unresponsive script)

Question

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?

---

EDIT 1: I started Firefox in Safe Mode (= without any add-ons): the problem still exists.

EDIT 2: I deleted some tab groups with many tabs (and added them as bookmarks) → this fixed the problem (it's still slow, but throws no unresponsive script warnings anymore). However, this question is still open, because I'd like to be able to use more tab groups.