I'll discribe one posible way to install SumatraPDF browser plugin for 32-bit Firefox on Windows, so that others who had unsuccessfully tried to do so can try one more option.
1. Downlaod version 2.5.2 and install it with plugin, then download and install latest version in same folder and keep the plugin.
2. Right click on Computer icon and choose Properties, and then choose Advanced system settings from left meny.
3. Open Enviroment Variables and create New User variable for PC with folowing properties:
Variable name: MOZ_PLUGIN_PATH
Variable value: <adress of folder where is plugin>
e.g. C:\Program Files (x86)\SumatraPDF\
or C:\Program Files\SumatraPDF\
4. Open Firefox and go to Tools, Add-ons, Plugins. Set SumatraPDF Browser Plugin to Always Activate, and all others plugins for viewing pdf fomat to Never Activate.
5. Now go to Tools, Options, Applications, find Portable Document Format (PDF) and choose Use SumatraPDF Browser Plugin (in Firefox).
6. If Use SumatraPDF Browser Plugin (in Firefox) is not offered close Firefox and go to C:\Users\<username>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile code> and delete mimeTypes.rdf, then open Firefox and do the step 5.
I hope that someone will find this helpful.