Can you make share preview link? It will help us diagnose butter.
I observe similar effects on my projects with different zoom settings. Please confirm zoom level in each browser instance.
Have you tested
If the The REM value in each browser is different, for example, chorme and opera might have a rem value of 17px, but firefox might have a rem value of 12px, then you may observe differnt font sizes between browsers.
I usualy set base font size in '<'body> all tag to set fixed base em font size and not rem font size
Also is each browser using the same responsive viewport? Did you confirm each browser is rendering the same viewport?
If you have 1280 1440 1920 breakpoints, you may potentially see different resoluts on different browsers depending on browser dpi settings and browser zoom settings