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How to Set Custom Zoom Levels in Firefox

It is possible to change the zoom level of web pages in Firefox browser in a couple of different ways. They include:

  1. Using Ctrl+Mouse Wheel
  2. Using the keyboard shortcuts Ctrl++ to zoom in, Ctrl+- to zoom out and Ctrl+0 to restore the default zoom
  3. Using the zoom controls available in the hamburger menu or the shortcuts under the View option in the menu bar.
  4. By setting a default zoom level inside the Settings / Preferences. The zoom level set here affects all pages.

In all the cases above, the available zoom levels are fixed to a handful of preset values with steps of between 10% and 100%. The maximum allowable zoom level is fixed to 500% and the minimum to 30%.

While these presets should suffice for most users, there are situations where you may want to define a custom zoom level with lower step values or that’s beyond the defined limits.

Fortunately, there are a couple of ways of achieving with or without using add-ons. Let’s check them out.

1. How to Add Custom Zoom Values

You can add a custom zoom values by going to about:config in the address bar and searching for the option: toolkit.zoomManager.zoomValues.

This option holds the zoom values that are set through the default zoom option in the Settings, the keyboard shortcuts, the hamburger zoom controls and the View menu shortcuts. It does not affect the zoom values set through the Ctrl+Mouse Wheel, which unfortunately cannot be overridden as it’s step value of 10% is defined in the source code.

The default values in this option are:

 .3,.5,.67,.8,.9,1,1.1,1.2,1.33,1.5,1.7,2,2.4,3,4,5

These values correspond to the zoom levels 30%, 50%, 67%, 80%, 90%, 100%, 110%, 120%, 133%, 150%, 170%, 200%, 240%, 300%, 400%, and 500% respectively.

Edit this option and add the custom zoom value you need in the sequence. For example, to add a zoom of 95%, just insert .95 between .9 and .1 and for 105% you’ll insert 1.05 between 1 and 1.1. Don’t forget to add the commas, then save the option.

To add 75%, 85% and 95% to the list use these values:

.3,.5,.67,.75,.8,.85,.9,.95,1,1.1,1.2,1.33,1.5,1.7,2,2.4,3,4,5

If you want zoom levels with a step value of 1% use the following values:

.01,.02,.03,.04,.05,.06,.07,.08,.09,.10,.11,.12,.13,.14,.15,.16,.17,.18,.19,.20,.21,.22,.23,.24,.25,.26,.27,.28,.29,.30,.31,.32,.33,.34,.35,.36,.37,.38,.39,.40,.41,.42,.43,.44,.45,.46,.47,.48,.49,.50,.51,.52,.53,.54,.55,.56,.57,.58,.59,.60,.61,.62,.63,.64,.65,.66,.67,.68,.69,.70,.71,.72,.73,.74,.75,.76,.77,.78,.79,.80,.81,.82,.83,.84,.85,.86,.87,.88,.89,.90,.91,.92,.93,.94,.95,.96,.97,.98,.99,1.00,1.01,1.02,1.03,1.04,1.05,1.06,1.07,1.08,1.09,1.10,1.11,1.12,1.13,1.14,1.15,1.16,1.17,1.18,1.19,1.20,1.21,1.22,1.23,1.24,1.25,1.26,1.27,1.28,1.29,1.30,1.31,1.32,1.33,1.34,1.35,1.36,1.37,1.38,1.39,1.40,1.41,1.42,1.43,1.44,1.45,1.46,1.47,1.48,1.49,1.50,1.51,1.52,1.53,1.54,1.55,1.56,1.57,1.58,1.59,1.60,1.61,1.62,1.63,1.64,1.65,1.66,1.67,1.68,1.69,1.70,1.71,1.72,1.73,1.74,1.75,1.76,1.77,1.78,1.79,1.80,1.81,1.82,1.83,1.84,1.85,1.86,1.87,1.88,1.89,1.90,1.91,1.92,1.93,1.94,1.95,1.96,1.97,1.98,1.99,2.00

The above values define zoom levels of between 1% and 200% however you’ll not be able to use any of the values below the default minimum zoom level of to 30%. You can however override this by searching for the option zoom.minPercent and setting its value to 1.

You can also define a value beyond the maximum 500% zoom level using the option zoom.maxPercent.

For example, if you want a max zoom level of 1000%, then set it to 1000 then add your zoom preset values as follows:

.3,.5,.67,.8,.9,1,1.1,1.2,1.33,1.5,1.7,2,2.4,3,4,5,6,7,8,9,10. 

This will add the zoom values 600%, 700%, 800%, 900%, and 1000%.

Note:

  • These min and max values will also apply to the Ctrl+Mouse Wheel zoom values, however the step value remains fixed at 10%.
  • You can have the zoom apply to only the text by using the Default Zoom option available in the settings and checking the option Zoom text only.

2. How to Set a Custom Global Zoom Level

A global zoom level affects all the elements of the browser, including web pages and the size of the tabs, menu and toolbars.

We can override this global zoom using the layout.css.devPixelsPerPx option inside about:config. The default value of -1.0 corresponds to a global zoom level of 100%. To increase it to let’s say 105% set its value to 1.05.

You can also decrease the global zoom level by using a value below 0. For example, use 0.9 to set the zoom level to 90%.

These changes are immediate, so you don’t have to restart the browser.

3. Using Add-ons to Set Custom Zoom Values

There are plenty of Firefox add-ons available for controlling the browser’s zoom level. A popular one that’s recommended by Firefox is Zoom Page WE.

This extension however is only suitable if you want to stick to the default preset zoom levels. I only include it here because it includes a host of useful features such as full or text only zoom modes, per site or tab zoom modes, font size override and a handy fit-to-width (auto) zoom option.

Comparatively, there are add-ons like Zoom for Firefox which allows us to define custom zoom values. This extension for example adds an icon to the toolbar which you can use to set zoom levels of between 1% and 400% using a slider or plus/minus buttons.

In the extension’s options, you can also define a zoom step value instead of the default 10%. Other options include choosing between the built-in or a CSS zoom engine, setting a default zoom level, and a list to define zoom levels per site.

Hopefully, one of these three methods will suffice for you to set custom zoom values.

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Kelvin Kathia

Kelvin Kathia is a writer based in Kenya that's passionate about sharing solutions to everyday tech problems. He's the founder and editor of Journey Bytes, a tech blog and web design agency. Feel free to leave him comments or questions regarding this post, or by leaving him a message on the contact page. If you found his content helpful, a donation is much appreciated.