All you should need is a Refresh of Firefox to clean up your configuration. What was optimal in Windows 7 may not even be supported in Windows 10, hence the trouble. See Refresh details and instructions here: https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings.

Your bookmarks and other user data contained in your profile should not be compromised by the transition. The main problem are the old settings, some of which may even originate from 2009 in the case of Windows 7 - it's a tall order to compensate for everything that has changed in 6 years and two operating system versions. This is easier done by Refreshing, so the browser can be adapted to your new operating system without being weighed down by half a decade of additions and transformations.

While Refresh does make automatic backups, it is a good practice to keep manual backups regardless. That way you are at minimal risk of losing your data.

PS. Here are the instructions for backing up and restoring your profile information: https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles.

My question was also asking for advice on Thunderbird. How can I can ensure when I change from windows 7 to 10 that I do not lose my inbox, folders, address book etc.

Also I have read a lot of questions in your support forum about Thunderbird freezing which do not seem to have had a satisfactory answer. How can I prevent this happening.

Firefox and Thunderbird are much the same when it comes to user data and profiles; I went into Firefox mode since I answer primarily Firefox questions on this (Firefox) forum. You can find the Thunderbird profile instructions here: https://support.mozilla.org/en-US/kb/profiles-tb.

"Thunderbird saves personal information such as messages, passwords and user preferences in a set of files called a "profile", which is stored in a separate location from the Thunderbird program files."

Does Thunderbird have the equivalent of the "Refresh" button in Firefox to make the process of changing to windows 10 from 7 a smoother transition? Also, in trying to back up my profile, I do not understand the instruction to "go one level up above your profile folder (xxxx .....) and select copy" What does one level up mean? There is nothing bove to click on.

By "level" the instructions refer to the directory structure.

For example in the path "C:\Example\Level1\Sublevel2", the Level1 directory would be one level up from Sublevel2 in the directory structure, and Example would be two levels up. You should see the levels and their respective directories in the address bar in Windows Explorer (the thing you use to navigate folders and files in Windows).

Thunderbird does not include Refresh functionality as far as I know. It probably has less need of it than Firefox, since TB is just an email client and not an internet browser, i.e. it is less complex a system.

Both are relatively painless to repair and restore as long as you have working copies of your profile folders.

If you run into trouble or want to make sure what to do, refer to the Mozilla KB for basic instructions and feel free to ask if you have further questions.