Thanks for the feedback and sources.
As @dlfuller stated, localStorage.clear(); does not work.
I opened the inspector(Right click > Q) and found the storage tab( You might need to active it in the inspector settings) and removed all my cookies related to the website by right clicking on a cookie from that site en clicking remove all form '...'. Then I logged in again and all problems were fixed.
But this should not happen, so there is probably something wrong in WordPress or in Firefox.
