Some websites have the terrible habit of manipulating what ends in the clipboard when the user copies some text. It’s common to append text such as “For more visit http://foo.bar” so when you paste your test it becomes something like this:
<yadda yadda yadda the text I copied yadda yadda yadda>
For more visit http://foo.bar
In some cases, that can be even a potential cyberattack
To disable entirely that possibility, when using Firefox, go to about:config and set dom.event.clipboardevents.enabled to false.
Unfortunately, in doing so, websites such as Google Sheets may break.
The alternative is to use the addon Luminous. Install it, add it to the toolbar, click on it, and select settings. Under default rules, find addEventListener and block copy. Proceed to website rules, add the domain you want to whitelist (e.g. docs.google.com and add the rule addEventListener/copy allowed.