Can you tell what application/window has focus when it has moved away from the previously active Firefox window? If you hold down the Alt key and press and release the Tab key, usually Windows will display a thumbnail of the window that had focus in the first position, and the window you're switching to in the second position. In the old days, that panel would display indefinitely until you released the Alt key, but I'm only seeing it briefly today, which makes it a little harder to investigate.

Hi, and thanks for responding so quickly. It happens when it is the only active window. I don't often have other browser windows open concurrently. Sometimes non-browser windows (Usually file explorer etc). I will monitor it further to see if additional tabs/windows make it better (or worse!) Edit. It has occurred twice while typing this response. This page open, MS Outlook Email page tab open (inactive) and one other (non Flash) webpage tab open (also inactive). Definitely no other windows open in this occurrence. At+tab shows desktop Sticky Notes open only.

As an experiment, could you slowly type every character on the keyboard and see whether one is triggering the loss of focus? If a plain character doesn't do it, try with the Shift key.

Also, do you use any security software that has an anti-keylogger feature? Such features divert your input through a separate program and sometimes have weird side effects in Firefox.

Tried a few times and couldn't repeat loss of focus via keyboard. (Damn! its just done it again...) Just now it lost focus just scrolling down webpage using mousewheel. (It just occurred twice during the time it has taken to type this question) ..Make that 3 times! I use ESET smart security and have done for some time, so I don't think that is the culprit. I'm going to do a clean reinstall of Windows on a new SSD in the next week or so, so lets park this item until the new machine is up and running. I can monitor it more constructively then. This seems to be getting worse and is now making FF unusable for me, so I really hope the clean install does the trick. The prospect of going over to Chrome or IE does not excite me one bit...

Could you test in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions, hardware acceleration, and some other advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement? If so, one of your extensions might be playing up.

I restarted in safe mode and it immediately lost focus. So we've established: Its not an add-on issue Its not a Flash issue because Flash has been uninstalled. I'll try uninstalling Firefox, downloading a fresh copy and re-installing. Please watch this space. Thanks again for your assistance and advice but as I said earlier, if this does not work I'll have to switch to one of the other browsers, because this is getting too tedious now. I'll try it again with a clean install of W7.

For the reinstall, could you try it this way:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent, corrupted, or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

(A) Download a fresh installer for Firefox 41.0.2 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(B) Exit out of Firefox (if applicable).

(C) Rename just the current program folder, either:

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(D) Run the installer you downloaded in (A). It should automatically connect to your existing settings.

Does Firefox work more normally?

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins

Otherwise, there's no reason to keep the old folder except research/curiosity.

Note that in current releases you can disable protected mode in the Flash plugin via the Plugins Add-ons page (about:addons)(Tools > Add-ons > Plugins). See:

• https://support.mozilla.org/kb/adobe-flash-protected-mode-firefox

You can check for issues caused by plugins and set plugins to "Ask to Activate" on the "Firefox menu button/Tools > Add-ons > Plugins" page.

• plugins are not affected by Firefox Safe Mode

Guys- I think I may have isolated the problem. I switched over to IE and that lost focus also. Then I noticed my USB Bluetooth adapter was plugged in and showing a blue light but was actaully disabled in Device Manager and in the notifications area. I enabled it and all seems to be well now. I'll keep this open for the weekend just to be sure, but meantime, thanks for your interest, help and advice. Its really appreciated.