You can modify this pref to have more steps for zooming via the keyboard (Ctrl and plus or Ctrl+ min), zooming with the mouse wheel is always in 10% changes. I use this list to have a lot of zoom values for making and inspecting a screenshot:

• toolkit.zoomManager.zoomValues;.1,.15,.2,.25,.3,.35,.4,.45,.5,.55,.6,.65,.7,.75,.8,.85,.9,.95,1,1.05,1.1,1.15,1.2,1.25,1.3,1.35,1.4,1.45,1.5,1.55,1.6,1.65,1.7,1.75,1.8,1.85,1.9,1.95,2,2.05,2.1,2.15,2.2,2.25,2.3,2.35,2.4,2.45,2.5,2.55,2.6,2.65,2.7,2.75,2.8,2.85,2.9,2.95,3,3.25,3.5,3.75,4,4.25,4.5,4.75,5,5.5,6,6.5,7,7.5,8,8.5,9,9.5,10

See also:

• zoom.minPercent (10)
• zoom.maxPercent (1000)

You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Whoa! That really solved my issue, I forgot that we could change the values in the config editor. Thanks for the help. Really appreciate it :)