To allow text to still be searchable as text, you could take all of your graphic elements into Photoshop to flatten and re-import back to InDesign as a bitmap image background.
Or, if you don't care as much about the selcectable/searchable text, a quicker route would be to open your PDF in Acrobat Pro, and File > Save As Other > Image > JPG. This will save each page as a separate JPG file to a folder you specify.
Then just click File > Create > Combine Files into a Single PDF and grab all of those JPGs you created.
Ultimately though, it sounds like a known Firefox viewer problem. Any 'solution' is really just a workaround to better support users on that browser.

Option 1

To allow text to still be searchable as text, you could take all of your graphic elements into Photoshop to flatten and re-import back to InDesign as a bitmap image background.

Option 2

Or, if you don't care as much about the selcectable/searchable text, a quicker route would be to open your PDF in Acrobat Pro, and File > Save As Other > Image > JPG. This will save each page as a separate JPG file to a folder you specify.
Then just click File > Create > Combine Files into a Single PDF and grab all of those JPGs you created.

Ultimately though, it sounds like a known Firefox viewer problem. Any 'solution' is really just a workaround to better support users on that browser.

EDIT

If using Option 2, you can adjust quality settings by clicking the Settings button below the filename in the Save As window:

...and then adjusting the quality settings (in red), and perhaps even the resolution settings (in yellow) if additional clarity is required:

Increasing these settings will obviously result in a larger file size, but will also make your text more readable. For maximum clarity, you'd want to go with Option 1 though. Although more time consuming, you'll get perfectly legible text at a fraction of the filesize compared to having rasterized text (like in Option 2).