Do you mean the title as shown on the tab and in the title bar? That is not possible AFAIK (WebExtensions do not work with about pages and chrome:// URIs).

• https://dxr.mozilla.org/mozilla-release/source/browser/components/sessionstore/content/aboutSessionRestore.xhtml

Yes.

I'm not looking for an addon, I was hoping it can be done using userContent.css since I've already customized this page with it, and what I need is to add a few letters to the title.

I've tried ::after selector but I can't get it to work (it works for other elements).

Is this for the title that shows in the tab?

Yes, I want to add a few letters to it, for example: "Restore Session" becomes "Restore Session - PF1"

Which is why I tried the ::after content.

Does this code in userChrome.css work for you?

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

.tabbrowser-tab[label^="Restore Session"] .tab-text{visibility:collapse}
.tabbrowser-tab[label^="Restore Session"] .tab-content:after{content:"Restore Session - PF1"}

The problem is that the window taskbar button is still showing only "Restore Session", which is why I think it must be done using userContent.css (or maybe a script loaded by it?).

Is there another way to do it?

Thank you for your help.