You will need to log in & tell your browser to save your password if you want to skip that step.
Many people say that you should not let browsers save your passwords because they are not secure enough.
As a minimum precaution, you should set up a Firefox primary password. If you don't then anyone who accesses your computer can read off all the user names & passwords your browser has saved for you.
Use a Primary Password to protect stored logins and passwords - Firefox Help
I have tested this [last year, I think] and
- I successfully read every username & password that had been saved by Firefox.
- the Primary password scheme does prevent people getting at your browser usernames & passwords. I tested that as well.
- the Primary password scheme will prevent you going straight through login. It will demand your PrimaryPassword first. This can be annoying.
- I use Edge to check my outlook.com inbox online. It was supposed to be secure enough to use without an added PrimaryPassword. I have not re-tested it but it certainly passed my tests last year [or was it the year before?]. And, by not having a PrimaryPassword & telling it to save my password, it lets me go straight to my Inbox.
Why are you talking about MSOffice [unknown version] & MSOffice 365?
I thought you were talking about accessing your outlook.com inbox online using your browser.
If you have MSOffice 365 you can use Outlook 365 to get straight there without using any browser at all.
Outlook.com and MSOffice Outlook [all versions] are completely different things.
Dinner time, back tomorrow,
Denis