Could you try this:

Create a new Firefox profile

A new profile will have your system-installed plugins (e.g., Flash) and extensions (e.g., security suite toolbars), but no themes, other extensions, or other customizations. It also should have completely fresh settings databases and a fresh cache folder.

Start up in the Profile Manager using the Run dialog (lower part of the Start screen or Windows key+R), type or paste the following and press Enter:

firefox.exe -P

Assuming the Profile Manager comes up, do not delete anything here

Click the Create Profile button, assign a name like May23, and skip over the option to move the folder location. Then start Firefox in the new profile you created.

Will it start?

If Yes, it probably indicates some corruption in one or more of your settings files. Some might still be salvageable. (More on that below the break.)

If No, the program files might be damaged. More on that in a second post.

The general help article on transferring settings files is here: Recover important data from an old profile.

If you want to import your bookmarks from your regular profile, here are the suggested steps for that:

You can use the Library dialog (Ctrl+Shift+b or "Show All Bookmarks") to access the Restore feature. The trick is knowing where to find your bookmark backups in your regular profile. After clicking Choose File... you can paste this into the file name space and press Enter to drill down to your profile folders:

%APPDATA%\Mozilla\Firefox\Profiles

The semi-randomly-named folder that has your last backup is usually the one ending with "default". Click into that one, then into the bookmarkbackups folder. Here you can pick the most recently updated file to restore.

Important: using Restore wipes any bookmarks in the Firefox profile you restore it to, and replaces it with the contents of the restored backup.

This method can resolve some issues:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

(1) Download a fresh installer for Firefox 38.0.1 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(2) Exit out of Firefox (if applicable).

(3) Rename the program folder:

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(4) Run the installer you downloaded in #1. It should automatically connect to your existing settings.

Any improvement?

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins