The following valid HTML5 document renders randomly (font and box sizes change in an unpredictable manner) just by reloading it in the browser (make sure to do shift-reloads so the browser cache gets reset). I see no reason why this should occur. What's going on?
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Some title</title>
</head>
<body>
<h1>Some heading</h1>
<form id="myformid" action="myformaction.php" method="post">
<label>label1: </label><input type="text" name="name1" required><br>
<label>label2: </label><input type="text" name="name2"><br>
<label>label3: </label><input type="text" name="name3"><br>
<input type="submit" value="Submit">
</form>
<p>This is a test</p>
</body>
</html>
This is something specific to your current setup, and without more information it is really difficult to say exactly why it is happening...
That said there are some obvious reasons as to why it could be happening.
To ascertain if this is the case, Start Firefox in Safe Mode. Safe Mode temporarily disables all extensions, uses the default theme, and turns off hardware acceleration. To do this; Click the menu button, click "Help" and select "Restart with Add-ons Disabled…" Firefox will start up with the Firefox Safe Mode dialog. If this fixes it then note it could be either an extension or hardware acceleration - to isolate which try step 3 below.
To ascertain if this is the case reset the zoom level when you notice the change in rendering. Click the menu button on the right. The customization menu will open and you will see the zoom controls at the top. Make sure that zoom level is set to 100%
To ascertain if this is the case, you can try turning off hardware acceleration to see if it fixes the problem. Click the menu button and select "Preferences". Select the "General" panel. Under "Performance", uncheck "Use recommended performance settings". Additional settings will be displayed. Uncheck "Use hardware acceleration when available".
Other than that providing screen-shots of the issue would go a long way towards isolating the cause of the rendering issue you are seeing.