Could you try disabling graphics hardware acceleration? Since this feature was added to Firefox, it has gradually improved, but there still are a few glitches.

You might need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.). Then perform these steps:

• Click the orange Firefox button at the top left, then select the "Options" button, or, if there is no Firefox button at the top, go to Tools > Options.
• In the Firefox options window click the Advanced tab, then select "General".
• In the settings list, you should find the Use hardware acceleration when available checkbox. Uncheck this checkbox.
• Now, restart Firefox and see if the problems persist.

Additionally, please check for updates for your graphics driver by following the steps mentioned in the following Knowledge base articles:

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Upgrade your graphics drivers to use hardware acceleration and WebGL

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

As I said in my initial question, I've tried disabling hardware acceleration and I have the latest drivers.

Another thing I noticed: if I disconnect the external monitor, then reconnect it, mouse movement is fine on both monitors until I move a program into the external monitor. Then mouse movement gets dodgy on both monitors again.

Check in safe mode

Try the Firefox Safe Mode to see how it works there. The Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• You can open the Firefox 4.0+ Safe Mode by holding the Shift key when you use the Firefox desktop or Start menu shortcut.
• Or use the Help menu item and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Don't select anything right now, just use "'Start in Safe Mode" To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shift key) to open it again. If it is good in the Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one.

Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

When you figure out what's causing your issues, please let us know. It might help other users who have the same problem.

I've already tried in safe mode. Sorry, that's what I meant by "without all of my add-ons".

Another thing I notice: if Firefox is on my laptop screen but doesn't have focus, then mouse movement is fine everywhere except where the Firefox window is.

hello draqonQ, i've seen very similar reports in this question & here. did this start with firefox 18? please also go to firefox > help > troubleshooting information & copy the info form the graphics section into a reply here - this might help debugging the issue... thanks

I'm afraid I can't be sure whether it started with Firefox 18 - I only got the external monitor last week. Graphics info is below:

Graphics

       Adapter Description
       Intel(R) HD Graphics

       Adapter Drivers
       igdumd64 igd10umd64 igdumdx32 igd10umd32

       Adapter RAM
       Unknown

       Device ID
       0x0046

       Direct2D Enabled
       true

       DirectWrite Enabled
       true (6.1.7601.17789)

       Driver Date
       7-31-2012

       Driver Version
       8.15.10.2827

       GPU #2 Active
       false

       GPU Accelerated Windows
       1/1 Direct3D 10

       Vendor ID
       0x8086

       WebGL Renderer
       Google Inc. -- ANGLE (Intel(R) HD Graphics)

       AzureCanvasBackend
       direct2d

       AzureContentBackend
       direct2d

       AzureFallbackCanvasBackend
       cairo

thanks could you also add the type of monitor & on what resolution it's running? and is there a spike in cpu usage in the windows task manager while the lags are occuring?

The internal screen is 1366x768. The monitor is a Dell Ultrasharp U2312HM running at 1920x1080 (native resolution). I'm using the latest drivers for it (although they're from 2011 I think).

ok thanks for all of this information - i've filed bug#841752 for this issue.