When logging into microsoft web apps using Firefox 140.9.0esr (aarch64) on my MacBook Pro, I get an error message reading "Bad Request - Header Field Too Long. HTTP Error 400. A request header field is too long." This error page loads after I sign-in using my workplace credentials on the sign-in page. For example, going to outlook.office365.com/mail. I've done some searching online already for solutions and have tried: 1) turning off enhanced tracking protection for this site 2) turning off all browser add-ons and extensions 3) regenerating the cookies4.sqlite* files in the Firefox Profiles folder 4) starting in "Troubleshoot Mode" The only thing that's worked has been to start Firefox in the mode where windows are not private, and then select "New Private Window" from the File menu. Within that window I can log in successfully to Microsoft Web Apps. BUT, when I turn on "Always Use Private Browsing Mode" in the settings and restart firefox, I get the "Header Field Too Long" error message even though the browser window is private. I'd like to run in Always Use Private Browsing mode. Ideas and help? TIA Dave My system: Firefox 140.9.0 esr (aarch64) MacBook Pro, Mac14.6, Apple M2 Max with 12 cores, 32 GB memory macOS 26.3.1