Many site issues can be caused by corrupt cookies or cache. In order to try to fix these problems, the first step is to clear both cookies and the cache. The Firefox cache temporarily stores images, scripts, and other parts of websites while you are browsing.
Note: This will temporarily log you out of all sites you're logged in to. To clear cache and cookies, do the following:

1. Tap the menu icon located at the top right corner. This is the icon with 3 bars. On older Android devices you'll have to press the hardware menu key and then tap More.
2. Tap Settings.
3. After that, you will be taken to the settings screen. In the settings screen, look under the section Privacy & Security and select Clear private data.
4. You will then be taken to a list of what can be cleared. Select the following 2 for deletion:
   • Cookies & active logins
   • Cache
5. After those have been selected, tap the Clear data button to actually clear the cache and cookies.

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Thank you for the suggestion. That is similar to what I read at Document Object Model (DOM) > DOM Storage > Storage location and clearing the data.

Unfortunately, it does not work.

At this point, the filesize of webappstore.sqlite-wml has grown by 2MB. Here is the size in bytes.

   4096   webappstore.sqlite
   12137 webappstore.sqlite-wml

A sqlite-wal file is a kind of temp file used to log database updates. The file should be automatically cleared when you exit Firefox, but you might need to use Quit for that to happen (Menu button > More > Quit).

Yes, the webappsstore.sqlite-wal (wal: Write-Ahead Logging) shouldn't be there after Firefox is closed. This sounds quite large to me for a temp file.

• http://www.sqlite.org/wal.html

You can try to delete both (all) webappsstore.sqlite files to make Firefox rebuild them.

Unfortunately, "Quit" does not remove these files.

This worked. I reduced the size of Firefox by over 14MB. Webappsstore.sqlite* were eventually created again, and are all about 32 or 97 bytes.

I wouldn't call this a solution, but it is a workaround. It requires knowledge of the filesystem and the ability to delete those files, and root access to the filesystem.

From this article, it sounds like the files can be deleted as needed. They will be re-created when needed.

http://kb.mozillazine.org/Webappsstore.sqlite#Deleting

Those suggested problems will not solve the issue here.

Bug 857888 has been opened to investigate the possibility of reducing the size of those databases.

Will removing webappstore.sqlite & webappstore.sqlite-wml work as a workaround?

Not advised as that will corrupt the profile.

And I certainly don't want to corrupt my profile, but it seems to be the only solution at the moment. webappsstore.sqlite-wal routinely grows to 8MB or larger. Sometimes the problem does clear up on it's own after 4-5 days, but I have no idea how to trigger the condition to clean it up.