The letters look a little larger and further apart in Brave, or is it the other way around. Could it be a difference in your zoom level? Easy to test by holding down the Ctrl key and tapping the + key to enlarge or the - key to reduce.

By default, Firefox uses Courier New as the Monospace font on Windows. You could change that to another font if there is one you prefer (such as Consolas or Lucida Console). It's probably easier to compare fonts in the Windows 10 font settings than by changing the font in Firefox, testing, changing, testing, etc.

Does this only happen with the developer tools or also on webpages that do not specify their fonts ?