https://sites.google.com/site/easylinuxtipsproject/2#TOC-Quicker-updates-for-Firefox

In the address bar of Firefox, type:

about:config

Press Enter and then click to accept the risk.

Then right-click anywhere on that page – New – String

Give the new string this name:

intl.locale.requested

Leave the value empty; this forces Firefox to follow the system locale.

Click OK.

Close Firefox and restart it.