Update:- I have found the cause of this problem. It an antivirus plug-in downloaded from my bank (RBS) called Trusteer Rapport. I uninstalled this and all is well.

We're sorry to hear that Firefox is crashing. In order to assist you better, please follow the steps below to provide us crash IDs to help us learn more about your crash.

1. Enter about:crashes in the Firefox address bar and press Enter. A Submitted Crash Reports list will appear, similar to the one shown below.
2. Copy the 5 most recent Report IDs that start with bp- and then go back to your forum question and paste those IDs into the "Post a Reply" box.

Note: If a recent Report ID does not start with bp- click on it to submit the report.

(Please don't take a screenshot of your crashes, just copy and paste the IDs. The below image is just an example of what your Firefox screen should look like.)

Thank you for your help!

More information and further troubleshooting steps can be found in the Troubleshoot Firefox crashes (closing or quitting unexpectedly) article.

you could also get in contact with trusteer's support in case you're interested in following up with them. they are usually very responsive in fixing any issues they cause... https://trusteer.secure.force.com/PKB/articles/FAQ/browser-crashes