That looks like a problem with fonts.
Is this with all websites/pages, some, a few?

Type about:preferences#general<enter> in the address bar.
Across from fonts and colors, press the Advanced button.
On the bottom, turn on Allow Web Sites To Choose Their Own.

Fonts Information - Detected via Flash http://browserspy.dk/fonts-flash.php?detail=1

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

If there is still a problem, Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Did this help?

While you are in safe mode;

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

How to disable Hardware Acceleration {web link}

This is Unicode in the 0980-09ff range that is used by Bengali scripts.

• https://en.wikibooks.org/wiki/Unicode/Character_reference/0000-0FFF

If you see little boxes with hex code then this means that you do not have a font installed that covers this Unicode range. You will have to install such a font. For me the FreeSerif system font is used on Linux.