Interesting. I only noticed now that shift-insert in Firefox doesn't use PRIMARY but CLIPBOARD. I'm so used to middle mouse button paste that I never noticed. This is probably due to Firefox not using a native graphical toolkit.

Don't they use Gtk2 & Cairo?

Chromium is slow to deliver the clipboard too (just containing text), so you press ctrl+v and wait for several seconds.

Chromium and some others take the clipboard with them when the app is Quit, suddenly the clipboad is cleared.

Really? I use chromium on the odd occasion and never have to wait.. nor have I ever noticed the clearing of clipboard..