Sorry to hear about this inconvenience. Hopefully we can help you track it down.

As a first step, could you create a new profile? It won't be customized, but at least lets you get into Firefox and start using the menu.

First, copy the following:

firefox.exe -ProfileManager

Then you can paste that into either the Windows Run dialog (hold down the Windows key and tap the R key) or the system search box (might be on the Charms bar on Windows 8.1) and press Enter to run it.

Firefox should display a small dialog with a Create Profile button:

Click that, choose a name like Dec2017, skip over any option to choose your own folder location, and complete the process.

Then highlight that profile on the list and click Start Firefox. In the next reply, I've give suggestions for looking for your old Firefox data.

Now... where is your data?

Could you first shut down and restart Windows? Sometimes at startup Firefox isn't allowed full privileges and your data is inaccessible.

If your data does not magically reappear, the most likely scenarios for complete data loss are:

(1) A Refresh started but the migration of data from your old profile to your new profile was incomplete, or
(2) Your old profile was locked, so Firefox ignored it and created a new one.

Here's how to investigate those two possibilities:

Did Firefox perform a Refresh?

Unless it was interrupted in progress, you should find a folder on your desktop named Old Firefox Data. Inside would be a semi-randomly-named folder with freshly updated files. Do you see anything like that?

If so, we'll suggest how to rescue key files or completely restore the earlier profile folder.

If not...

Search for Other Profile Folders

Does the internal Profile Manager list your immediately previous profile folder?

To check on that, inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

This page should list all the profiles Firefox is currently aware of. If you see a profile that is not currently set as default that looks promising, scroll down to it and click the Set as default profile button below that profile, then scroll back up and click the Restart normally button. (There are some other buttons, but I think those are still "under construction" so please ignore them.)

Firefox should exit and then start up using the other profile. Is that the right one?

Based on your original error message, one of these profiles is missing. If you end up restarting into a missing profile, you'll need to use the external Profile Manager to switch to a working profile.

If it's not there...

Did Firefox orphan your old profile folder?

Here's how to check whether there are additional profile folders in the standard location in addition to the one you're using. Start by opening your current Firefox profile folder using either:

* "3-bar" menu button > "?" button > Troubleshooting Information
* (menu bar) Help > Troubleshooting Information
* type or paste about:support in the address bar and press Enter/Return

In the first table on the page, click the "Open Folder" button, to launch a new window listing various files and folders in Windows Explorer.

In the Windows Explorer address bar, click the capitalized word Profiles to move up one level in the folder hierarchy. More than one folder here by any chance? Do the files in those folders look like they were last updated around the time of the update?

If you've got any promising looking folders, we can discuss next steps to recover use of them, or at least your bookmarks.