I'm having the same issue with mine, the media plugin cef won't load, and it's preventing me from actually logging in. I have no idea how to fix this aside from a full reinstall of my viewer..
This is what I pulled from my Help>About:
Firestorm 5.0.7 (52912) Jun 13 2017 03:57:58 (Firestorm-Releasex64) with OpenSimulator support
Release Notes
CPU: AMD Phenom(tm) II X6 1090T Processor (3199.99 MHz)
Memory: 10232 MB
OS Version: Microsoft Windows 7 SP1 64-bit (Build 7601)
Graphics Card Vendor: ATI Technologies Inc.
Graphics Card: AMD Radeon (TM) R7 370 Series
Windows Graphics Driver Version: 8.17.0010.1433
OpenGL Version: 4.2.13417 Compatibility Profile Context 15.301.1201.0
RestrainedLove API: (disabled)
libcurl Version: libcurl/7.47.0 OpenSSL/1.0.1i zlib/1.2.8
J2C Decoder Version: KDU v7.9.1
Audio Driver Version: FMOD Ex 4.44.61
LLCEFLib/CEF Version: 1.5.3-(CEF-WIN-3.2526.1347-32)
LibVLC Version: 2.2.4
Voice Server Version: Vivox
Settings mode: Phoenix
Viewer Skin: Vintage (Classic)
Window size: 1920x1058 px
Font Used: Deja Vu (96 dpi)
Font Size Adjustment: 0 pt
UI Scaling: 1
Draw distance: 128 m
Bandwidth: 1500 kbit/s
LOD factor: 4
Render quality: Medium-High (4/7)
Advanced Lighting Model: No
Texture memory: 512 MB (1)
VFS (cache) creation time (UTC): unknown
Built with MSVC version 1800
July 14 2017 19:19:10 SLT
If someone can please help me out, I would appreciate it very much, thank you!