To resolve login difficulties on Firefox, follow these steps:
Click the Hamburger icon (Menu) and select Options.
General page appears.
Search for 'Cookies'.
Search Results highlighting the word 'cookies' appear.
Click Manage Data.
In the Manage Cookies and Site Data pop-up, search for ‘Zenoti’.
Select the site, click Remove Selected and click Save Changes.
The Removing Cookies and Site Data pop-up appears.
Close and reopen your browser.
Login to Zenoti again.
Refer to the link pointing to Firefox help portal, for additional information on enabling and disabling cookies in Firefox.