I'm not seeing anything usual.

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

You may have accidentally zoomed web page(s). Reset the page zoom on pages that cause problems.

• View -> Zoom -> Reset (Ctrl+0/Command+0 (zero))

• http://kb.mozillazine.org/Zoom_text_of_web_pages

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

Clear the cache and remove the cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Clear the cache":

• Options/Preferences -> Advanced -> Network -> Cached Web Content: "Clear Now"

"Remove the cookies" from websites that cause problems.

• Options/Preferences -> Privacy -> "Use custom settings for history" -> Cookies: "Show Cookies"

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

1. The Firefox zoom value is 100% on both platforms.
2. I pressed Ctrl + F5 in Firefox for Ubuntu.
3. I cleared the cache and cookies in Firefox for Ubuntu.
4. I launched Firefox for Ubuntu in safe mode.

None of these steps solved the problem. Here are the screen shots.

The problem seems to be that some margin and padding values are specified in 'ex' (x-height) units and that seems to be different on Linux (3.5ex -> 35px). So you would have to use a different unit.

• https://developer.mozilla.org/en-US/docs/Web/CSS/length

Once upon a time, it was considered bad form to use pixel dimensions, but now that everyone has "full zoom" to resize things proportionally, I think you could safely use them if necessary to get the consistency you want.