The zoom level steps in Firefox are a comma-separated list of decimal values held in about.config (toolkit.zoomManager.zoomValues). The default on mine seemed to be
0.3,0.5,0.67,0.8,0.9,1,1.1,1.2,1.33,1.5,1.7,2,2.4,3
so to get a more fine grained series I changed this to 0.1 steps:
0.3,0.4,0.5,0.6,0.7,0.8,0.9,1,1.1,1.2,1.3,1.4,1.5,1.6,1.7,1.8,1.9,2,2.1,2.2,2.3,2.4,2.5,2.6,2.7,2.8,2.9,3.0
I had to restart Firefox for this to take effect. Following this I was able to see that when zooming a message in Gmail the horizontal scroll bar previously enabled vanished out of the bottom of the Firefox window at 220% zoom level. If I then hit F11 for full screen Firefox I was able to get up to 240% still with a horizontal scroll bar. At 250% it vanished.
Note: my display resolution is 1920x1080
I don't know if these whole-screen grabs will show up properly:
At 240% zoom full screen:
For comparison, here is 100% zoom: