We have been developing a SaaS app for two years now and the most serious issue is that Firefox Nightly shares the profile with the stable build. Sounds harmless, but it is not if you have installed Nightly and stable and the website uses indexeddb! The indexeddb, once modified in the Nightly, cannot be opened in stable build anymore. So you are trapped in Nightly forever if you access websites with indexeddb. The only way back is to completly erase your profile. That's why we actively warn users not to install Nightly on their system.