I am aware that I can copy my Thunderbird profile from my desktop to my laptop pc. However, why would I need to do that if all my Mail-Accounts are using the IMAP protocol? Am I missing something?
Your email account information is just one of the many things stored in the profile. The profile contains everything that makes your Thunderbird installation different from the bare software and default settings. If you look in the profile, you'll see the range of content.
I would even prefer two separate Thunderbird profiles...
If the profiles will be very similar, copy the profile, then modify what you want.
Can I safely use two different Thunderbird profiles to access the same Mail-Accounts as described above?
You can access IMAP accounts from as many computers and profiles as you want. Each profile will sync with the IMAP servers independently of other profiles.
Is there a way to import the addressbook on the servers?
If you do all of you emailing in Thunderbird, your address books will be stored in the profile. Copying the profile will copy the address books.
If the only information in the webmail addressbook is what it captured from messages involving contacts that Thunderbird also processes messages for or from, Thunderbird may well already have it. Compare the addressbooks; you might not need to do anything.
But if you also do some emailing from the IMAP provider's webmail client, its addressbook could contain contacts or added details not in Thunderbird. If you're talking about just the initial Thunderbird setup, yes, that would typically be done by importing the addressbook.
How to do that depends on the IMAP provider. IMAP only deals with messages. The email provider chooses a format to use for their webmail client's addressbook. There are a number of formats commonly used. Worst case, you may need to have the webmail client export the addressbook in a format Thunderbird knows how to handle, then import that into Thunderbird. There's a lot of online guidance on how to import addressbooks from specific email providers. This article by Mozilla, Sharing address books, will give you some general background (the article is a little dated, so you'll want to verify the utilities it mentions, and there are likely newer ones).
Thank you for this detailed answer! – LCsa – 2019-09-06T20:30:56.907