The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information. Note: This will cause you to lose any Extensions, Open websites, and some Preferences.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

Did this fix your problems? Please report back to us!

Tylerdowner, hi.

Thanks for your response, but remembering the cause of a previous attachent problem that many Firefox users were having at around the time of FF13 related to the network.http.spdy.enabled setting to 'true' I decided to check that setting again in about:config.

To fix my problem at the time I, like others, changed the SPDY setting from 'true' to 'false' and this I found had not been automatically changed with later versions of FF. I did find, however, that there were now two other SPDY settings for v2 and v3, which were set to 'true'. In case these were conflicting with the first setting on 'false' I changed this latter to 'true' also. This done I returned to Gmail and tried to attach some files, firstly in the normal way by clicking on attachments and selecting a file from the dialogue box, which worked (previously on 'true' this had not worked); secondly, I tried attaching using drag & drop, which now worked. I also found that drag & drop worked in Yahoo too.

So, could a conflict between the SPDY versions have been the cause of the problem when network.http.spdy.enabled was set to 'false'?

If the problem occurs again I will let you know. (I have attached a snapshot of the current settings.)