Since a few months, I have noticed a nasty issue with pasting on some websites. For example, when I try to paste text in the body with CTRL/CMD+V on the web interface of my email provider (mailbox.org), the cursor is moved to the end of the message but no text is copied. Right-click > paste does work, and no issue for the recipient and subject fields. The issue is only when writing emails in HTML; plain text works fine. Another example, when editing a protocol on protocols.io, hitting CTRL/CMD+V leads to a blank page, which needs to be refreshed (but the text has not been pasted). There is no option to do it via right-click > paste there. Same issue when submitting a manuscript on PCI Paleontology. All these examples are specific to Firefox (I have the latest version installed); it is reproducible both on macOS 10.15 and Win10. It doesn't seem to be due to add-ons (I tried to deactivate them one by one and nothing changed). There is no problem with Chrome-based browsers (e.g. Iridium) or Safari. Is it maybe some configuration or something? Any idea?