I guess this is not possible ...

I'm not aware of a simple way to extend the time that notifications linger. Here are a couple of ideas:

(A) Firefox can send notifications to the Windows action center

I think there were some bugs with notifications getting lost, so if you can't afford to lose any, you might not want to experiment with this one. But since you are missing them anyway, maybe:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste alerts.useSystemBackend and pause while the list is filtered

(3) Double-click the preference to switch the value from false to true

(B) Websites can require that notifications stay open until you close them

By default, Firefox ignores this requirement, but you can turn it on and see whether that helps with the sites you care about. I don't think this is applicable to the Windows action center, but I haven't tested.

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste dom.webnotifications.requireinteraction and pause while the list is filtered

(3) Double-click the dom.webnotifications.requireinteraction.enabled preference to switch the value from false to true

ty, I guess I should also increase dom.webnotifications.requireinteraction.count (currently set to 3) if I want to keep all open?

I guess (B) does only work for websites? I need it for an extension (Feedbro). I tried your idea but the notifications still disappear

Hmm, I don't know how extension notifications work. I wonder whether the extension has an additional Action option to star an item or add it to a list so you can find it if you miss the notification?