This is VERY weird. I am running Firefox 126.96.36.199 on an up to date version of Windows XP. I've been using FF for a year or so, so I'm very familiar with a lot of its quirks, but this one takes the cake.
Recently, I noticed that opening a new Window (^N) was VERY slow (> 1 min)! Opening a new Tab (^T) was still fast as ever. So I figured I'd recently installed a plugin that was causing the slow down. So first, I invoked "firefox -profilemanager" and startup up a clean profile. As I suspected, new Windows now opened just as fast as new Tabs.
So next I went into my default profile and started disabling plugins. Imagine my surprise when I disabled the last plugin and restarted and found that new windows were STILL opening slowly. I was truly mystified. One other unusual thing was that the frame of the window would appear instantly, but it would take a long time for whatever page to load. Even stranger was that Windows Task Manager was telling me while waiting that neither the CPU nor the hard drive were working hard at all.
This made me suspect network issue, despite the fact that the exact same URL, when opening in a new Tab was fast, but when opening in a new Window was extremely slow. I don't know what made me try it, but I turned on my corporate VPN and then tried opening a new Window. Voila! The new Window opened just about as fast as a new Tab! This makes NO sense to me at all.
So here is a summary of the bizarre symptoms:
1. New Tabs alway open fast, no matter what the URL or whether the VPN is active or not, and regardless of the profile I am using (default or clean).
2. New Windows in a clean profile always open fast no matter what the URL or whether the VPN is active or not.
3. In my default profile, new Windows open VERY slowly unless the VPN is active.
This is driving me crazy. Help! I already figure I can start from a clean profile (or a reinstalled firefox) and add my plugins one by one, but I'm looking for a less tedious solution to this mystery -- one that truly explains it.