If you find that you are no longer able to copy/paste text to or from the viewer, the cause may be…
This appears to affect only Firestorm 4.4.2 and later versions. However, the issue is not restricted to Firestorm or even SL viewers in general; see here for example.
If you find yourself unable to copy/paste using Firestorm, chances are good that you're using the Webroot anit-virus. If so, the issue is with “Identity Shield”.
Note that this change may affect performance when using WASD keys for movement. If you notice your movement using WASD keys is not smooth, but moving with Arrow keys is smooth, change the setting from Protect to Allow, or try the suggestion below
Should this not fix the problem:
Webroot bug report: Copy-Paste-Stops-Working-in-some-applications
General Webroot issues: Webroot and SL Viewers
With thanks to West Habercom for the Webroot info, and to Rhiannon Skinstad for additional info.