You appear to be using windows so please review these instruction on creating a screen shot of your problem and reply with same so we understand what it is you are seeing.

https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem#w_create-a-screenshot-on-windows

My guess is the issue is something extremely simple, as these sorts of issues are usually folk just missing something obvious, but without seeing what you see guessing is all I can do.

Here is the screenshot as an attachment

re :A few days ago my wife clicked to open an email and the whole Thunderbird page changed so that it is now almost un-usable.

Thanks for image, it shows your wife has opened loads of emails each in a separate tab. That can occur if you have 'Threads' enabled and you click on a top level collapsed thread. It also looks like wife has also closed the Folder Pane. You could simply keep clicking on the < icon on far left to get back to the original tab, select it and then Right click on the tabs and select 'Close other tabs'. But as I do not know how many got opened, that might take a while and a lot of clicking.

Do this: Right click on the tabs and select 'Close other tabs' to exit most. On far left, it should get the original Folder you selected in a tab - select it. Now make sure the Folder Pane is visible. View > Layout > select 'Folder Pane'

If you do not normally use 'threading' then I suggest you switch off 'threads'. Select 'Inbox' View > Sort by > select 'Unthreaded'

Threading is not enabled and right clicking on the open tabs to close the tabs is not doing anything. If I go all way to the left next to all the open tabs I can find the link to all my emails but it is not in the format I am used to seeing. It looks like lots of other people have been having problems since the last automatic update so it seems that I am not the only one having this problem. Bruce

Threading is not enabled and right clicking on the open tabs to close the tabs is not doing anything.

Do you see the menu? Is it active? or disabled? I know it may sound obvious to you, but I really have no idea what you are seeing so statements that something just does not work is something I can not visualize as it works for me.

Do you have an antivirus product installed? What is it? Personally over the last 10 years I have seen more issues caused by antivirus in mail and I have ever heard of them fixing. Mostly they just corrupt data or block the saving of settings or connection to the mail server. Sometime all at once.

You are aware they are tabs and clearly not a reliable way to view all your mail, hence your current issues. Thunderbird uses a list or table view not a horizontal menu of mail. It is not intended to be uses as such and is not tested as such. Your choice to do it the way you are by having every single email open is not something that is all that relevant in the context of support. All we can do is explain how it works and we have done that to the best of our ability.

Perhaps you would like to make a suggestion to the developers that they build some sort of horizontal mail menu in the future as an enhancement. You can do that at this link. https://connect.mozilla.org/ it is also accessible from the Thunderbird help menu as "Share ideas and feedback". But it is not something I intend to invest time in.

Note this is a peer support forum. Thunderbird developers do not come here looking for ideas, feedback or suggestions. They use the link they built into the product for that function.