I have uMatrix set to delete cookies after 15 minutes. Is there a way to keep cookies from a specific domain from being deleted?
no, either all or nothing. uM is the wrong way for you. try a combination of firefox settungs (keeping), "cookie autodelete" and uM to block (read-out) cookies
Yes, just don't block them, i.e. set an allow rule such that cookies are not blocked for the specific domain.
oh, thats new for me. uM only delete cookies if the site is not whitelisted -> green field in the matrix?
The setting is "Delete blocked cookies after...", so it applies only to cookies which associated cell in the matrix is reddish.
