Evidently you are using a mobile phone. You need to ask below although someone may be able to answer your question here.

https://support.mozilla.org/en-US/products/mobile

You can set layout.css.devPixelsPerPx to 1.0 (default is -1) on the about:config page. Adjust its value in 0.1 or 0.05 steps like 1.1 to enlarge or 0.9 to reduce until icons/text looks right. You may need values above 2.0 if you have a high resolution display but make sure not to use values too large or too small.

• modifying layout.css.devPixelsPerPx affects user interface and webpages (global zoom)

Firefox has a Zoom section in Settings to set the default zoom level for webpages.

• Settings -> General -> Language and Appearance -> Zoom

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

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