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Browsers & Npapi

Occasionally I need to try out SumatraPDF NPAPI (Netscape Plugin Application Programming Interface) embedded in a portable browser (mainly out of interest about changes in browser functions) introduced in 1995 most recent browsers dropped support for it two decades later in 2015.

The browser plugin alows you to view online .djvu .xps etc the same way as an online .pdf

here is a djvu file in the latest version of Seamonkey 2.49.3 (May 2018)

Here is my list of recent observations (Any feedback from others still using it ?)

Seamonkey & Firefox release 52ESR for XP are current best all round means of running SumatraPDF browser plug-in on all versions of windows (XP to 10x64)
[Later edit] This support was end of life Last quarter 2019

Waterfox current lists the PDF plugin but I had problems trying to run it for some formats
[Later edit] Resolved, However this support is retired to Classic version in 2020 so may be depreciating
Current version works with both djvu and xps but retains waterfox for pdf viewing

midori 0.5.11 (2015) and Otter 0.9.99rc9 (2018) both work but frustratingly have cross platform issues with rendering the toolbar ??

Slimjet 10.0.13 (Chromium 50 for XP) Works as expected when Chromium Viewer is disabled
Vivaldi 1.10 (Last XP version) Listed plugin but not working so can only use inbuilt PDF Viewer. Not recommended


actually it works with firefox 32bit, but not 64bit…


@rfvv Cheers can you confirm which version you tested ? since more recent ones supposedly dropped support. the current list on seems to exclude any current firefox


Hello, i’m having the same problem.
Win 64bit + Opera 11.50 (<- not my decision).

Ain’t there a way to run it in a 64bit OS?


was dropped by most browsers in 2015 and AFAIK that included Opera
" Opera dropped support with version 37 in May 2016"

and at the time Firefox version 52esr was the last NPAPI resort

SeaMonkey (a firefox variant was the last man standing back then)

Due to the overall lack of legacy support I have not tested any browsers recently

I am downloading an old version of Opera to test for 64 bit support but first off the installer suggests Opera is 32bit from help about on a Win 10x64 I see
About Opera
Version information Version 12.14
Build 1738
Platform Win32
System Windows 8

tried using that but hit the usual problem of bitness mismatch so where do I find an old opera x64 that works on windows 10


But this opera is 32bit.
only the OS is 64bit.
I’ve tested sumatra 32bit and 64bit - none works.

FoxIt reader, however does work. But it’s too bloated and not opensource :frowning:


Browser bitness is key 32 bit browser needs 32bit SumatraPDF plugin
ensure you have a valid registered copy of npPdfViewer.dll in the installed 32 bit SumatraPDF directory If it works for one NPAPI browser then it should work for any other enabled browser. But am often strugling to find a good working mix. So am manually adding 2.5.2 plugin to previous version 3.1.2 x32
I have to go through windows administrative contortions to get reasonable results.

OLD 32 bit opera 12 can see a djvu document no problem

Note I have tested if it will work with 3.2 x32 and it appears it can (unproven since I have several versions active however the red background color is one I use in 3.2 and could exhibit a known 3.2 bug if I attempt to modify the toolbar) which was suprising as it was not compiled with version 3.2 in mind. Note that I had to reregister the plugin AFTER installing 3.2

Out of interest I tested pre-release 32bit v3.3.12654 with result of an obvious change in the toolbar HOWEVER “save as” icon no longer works, but I dont think we should raise a bug report for an unsupported usage.


To answer the original question
It appears for 64bit windows that WaterfoxClassicPortable 2020.0x (197 MB) may be one of two of the last 64bit contenders. The other seems to be PaleMoon (111 MB)

And 32bit Seamonkey Version 2.49.5 (128 MB) from 2019 (~= Firefox 52.9 ESR) was the Last one with known NPAPI support so PaleMoon 32bit Portable (100 MB) is now possibly the best contender there too.


When you moved the wiki to the main site, you forgot about the plugin

can you upload again the plugin


Its not forgoten just totally depreciated (unsupported)
wayback link

32bit was available in version 2.5 (2014) and dropped in 3.0 (2014)
for 64bit you will need a developers build.

Note it is working here and above with build 3.3.xx but there have been changes that mean some features such as annotation cannot be expected to work, since they were not available or not intended to be accessible when the plugin was developed. DjVu rotation is a current bug as also seen in the latest desktop reader.
Best to combine with a stock 3.1.2 release.


For those interested in a working SumatraPDF Web Browser (You can still type web URLs into the FileOpen box in the desktop app) so as to view on-line DjVu (O)XPS in addition to PDF.

My suggestion is install 32 bit sumatra 2.5.2 with browser option selcted
Update/upgrade that to 3.1.2 32bit (it should ask to do so itself, but beware any later offer to upgrade to 3.2.1)
Use PaleMoon Portable (it should be 32 BIT to match SumatraPDF, but may sometimes work with x64 browser but 64bit plugin can not work with 32bit browser)
check the Plugin is auto detected (this the 64 bit one but 32 bit should be the same)

To use as a portable version copy, the full SumatraPDF from installed PC (3-6 files) needs to be copied to a USB you will also need to write your own deregister from this PC and reregister for next PC (or simply install sumatraPDF on the new PC then uninstall when done, its quick enough for a max of half a dozen files)


And since nobody is asking why on earth would you want to keep a really useful PDF plugin when there are others suited to browsers. The answer is that you can have highly customised layouts of multiple documents in one Tab, so no jumping about between windows just click in the frames