Hi Craig, I've never noticed a limit on the number of windows I can open, but I generally open 10 or fewer windows and open tabs within the windows instead. Sometimes that may be one tab, or in one window I currently have 21 tabs open.

Now... Firefox fits a limited number of tabs on the tab bar because Firefox has a minimum tab width designed to keep the tabs somewhat readable. Once the minimum width is reached, when you open more tabs, arrow buttons should appear at both ends of the tab bar so you can scroll along the bar to see the other tabs. Also, you should see a drop-down list of all the tabs in the window (see attached screenshot). Are you not getting those controls, or did you want to let the tabs get smaller?

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

You have the 64-bit Firefox version, so there shouldn'be a problem to open a lot windows assuming that you have enough physical memory installed. You may also have to increase the size of the Windows page file (virtual memory).

Do you have enough free memory?

See also the about:memory page.