Another option for you to install a firefox browser is through here
Open any of your existing browser on your phone.
go to this link
and at the bottom, you will see the box for Mobile
click the download icon for
21MB | apk
or you can access this link to be directed on the download link
after the download has been completed,
go to your My files -> download
click the fennec-19.0a1.multi.android-arm-armv6.apk
then click install, and wait until installing is done.
That's it. Hope it will help.